Article: How To Turn a Reliever Into a Starting Pitcher: Inside Baseball’s Newest Pitching Trend

[Maddie Landis]

What does an elite reliever need in order to have a chance of succeeding as a starter? And how might the Red Sox benefit from this trend?

Relievers are us

ually perceived as failed starting pitchers. Being demoted to the bullpen is sometimes the last chance for pitchers to prove they belong in the majors. Some pitchers discover newfound success in the bullpen. Zach Britton, Andrew Miller, and Dellin Betances were highly acclaimed starting pitching prospects who became standout relievers only after failing to live up to their hype. Nick Pivetta was demoted to the bullpen after an abysmal start to the 2023 season. He returned to the starting rotation after improving his form.

Typically, managers usually turn to Triple-A starters for rotation depth, but recently, it’s become more common to see relief pitchers transition into starters. This year’s postseason features several former relief pitchers t

aking the mound. The following metrics help identify a reliever pitcher’s potential to transition to a starting pitcher successfully: 

1. Previous Experience as a Starter: Many relievers started in high school or college, transitioning to the bullpen after being drafted to meet their team's needs. Converted starters must be accustomed to pitching 5+ innings per outing. Possessing a pre-established five-day pitching schedule is crucial for a starting pitcher’s recovery and preparedness for their next outing. 
2. Multiple Elite Pitches: Stuff+ considers the physical characteristics of a pitch, such as release point, velocity, spin rate, and movement, and then gives it a grade, with 100 being league average. A successful reliever-turned-starter should have at least two pitches that exceed that threshold. Two elite pitches are a solid base for a starter’s repertoire.
3. Solid Control: Stuff+ has its limitations because it doesn’t measure command or control. While relievers can often get by with less precise control, starters need better command to navigate the lineup two to three times. Location+, zone rate, first strike rate, and K-BB% are good indicators of a pitcher’s ability to consistently find the strike zone. 

 

Applying these metrics to pitchers such as Seth Lugo, Sean Manaea, and Michael King serve as a baseline test for identifying potential conversion targets among MLB relievers. 

Seth Lugo, RHP Kansas City Royals

The Mets drafted Seth Lugo in 2011. He worked his way through their farm system as a starter, made his major-league debut as a reliever in 2016, and was later moved into the starting rotation. He posted a 2.67 ERA, 15.2 K%, and 1.15 WHIP as a starter in his first major league season. The team moved him back and forth from the rotation to the bullpen to accommodate Zack Wheeler’s breakout. Despite being typecast as a relief pitcher, Lugo always viewed himself as a starting pitcher. 

After the 2022 season, Lugo entered free agency. He signed a one-year $7.5-million contract with the Padres, who offered him a spot in their starting rotation. In 2023, he started a career-high 26 games across 146.1 innings, recording a 3.57 ERA, 28.5 K%, and 1.20 WHIP. Lugo possesses a robust repertoire that features a four-seamer, sinker, curveball, slurve, slider, cutter, changeup, sweeper, and splitter. 

Prior to Lugo’s move to the rotation, Stuff+ graded his 2022 slider (117) and 2022 curveball (107) as above average. Although the pitch's performance didn't grade out that well, it's not hard to see what the models liked about it. At 88.9 mph, it was faster and tighter than the average slider.

Hitters embarrassed themselves when faced with Lugo’s curveball. It very nearly had the highest spin rate in baseball, and its 17.2 inches of induced drop put it in the 97th percentile of all curveballs. The pitch yielded a 30.1% whiff rate, which was right around league average, but the opposition batted just .159 against it.

Lugo doesn't just have a plethora of pitches in his arsenal, he has command of them. He posted a 101 Location+ in 2022, locating 43.6% of his pitches in the strike zone and running a 61.4% first strike rate.  

Lugo made 33 starts for the Royals this year and he finished the regular season as one of the best pitchers in the league. His 206 2/3 innings pitches were second in baseball. His 3.00 ERA ranked 10th among qualified starters, and his 71 ERA- (which accounts for the pitcher's league and home park) ranked fifth. In all, his 4.7 WAR were sixth among all pitchers. On October 2, Lugo took the mound for the Royals against the Orioles in Game 2 of the American League Wild Card Series. He gave up one home run and struck out six batters across 4 1/3 innings, ushering the Royals to the ALDS. 

Sean Manaea, LHP New York Mets

Sean Manaea was drafted by the Royals in 2013. He’s worn various hats during his time in the majors, working as a starter, middle reliever, and long reliever. Manaea made 10 starts and 27 relief appearances with the Giants last season. He signed with the Mets in January, and after a strong performance in spring training, he was expected to be the Mets No. 4 starter. Over the past two seasons, Manaea’s arsenal has primarily consisted of a sinker, sweeper, changeup, four-seamer, cutter, and slider. 

Manaea’s arsenal is an enigma. According to Stuff+, most of his pitches are below league average. Manaea’s 2023 sinker was his only above-average pitch, graded at 121, but he hardly used it at all. Although they didn't impress the pitching models, Manaea’s four-seamer, slider, and sweeper recorded competent whiff rates and putaway percentages. His 93.6-mph four-seamer was deceptive, featuring much more arm-side run than the average fastball. 

Throughout his career, Manaea has also had solid command, running a 101 Location+, along with above-average zone and first strike rates. Manaea “passed” two out of three of the baseline test metrics. How did he fare as a full-time starter in 2024? The answer is complicated. On July 25, Manaea watched Chris Sale ca rve up the Mets, and he decided to pitch more like Sale. Just like that, he dropped his release point, moved to the first base side of the rubber, and became an unhittable, cross-firing, sinker-slider, sidearm lefty like Sale. Over his next 10 starts, he went 5-1 with a 2.49 ERA and averaged well over one strikeout per inning. He shelved his four-seamer, cutter, and regular slider completely, and threw his sinker more than 60% of the time in September.

In all, Manaea had a career season. In the regular season, reaching double-digit strikeouts in four games and throwing a career-high 181.2 innings across 32 starts. He ran a 3.47 ERA, 87 ERA-,  and put up 2.8 WAR. Kodai Senga, the team's primary ace, was sidelined with various injuries this year, and Manaea stepped up to fill his shoes. Now, he’s spearheading the Mets postseason run. 

Michael King, RHP San Diego Padres

Carlos Rodon and Luis Severino’s injuries derailed the stability of the Yankees' 2023 starting rotation. Michael King, a Yankees relief pitcher, advocated for himself and proposed that he could slot in the rotation. King initially came up as a starting pitching prospect. During his first couple of seasons in the majors, King bounced back and forth between the bullpen and rotation. He was a dominant reliever, but couldn’t achieve the same level of their success as a rotation pitcher. From 2019 to 2022, he had a 6.52 ERA and 4.94 FIP over 10 starts, and a 2.85 ERA and 2.96 FIP over 56 relief appearances.

In late August 2023, King transitioned to the Yankees rotation. He took a momentous step forward and pitched 38.1 innings across 8 starts with a 1.88 ERA, 2.47 FIP, and 31.3% strikeout rate. Because of his strong finish, the Yankees anticipated that he would be part of the 2024 rotation. However, King was part of the blockbuster trade package that brought Juan Soto from San Diego to the Bronx. That same day, San Diego GM A.J. Preller confirmed that King would be part of the Padres 2024 rotation.

King primarily relies on his sinker, changeup, four-seamer, and sweeper, throwing them at roughly the same rate.

In 2023, Stuff+ graded his four-seamer(106) and sinker (109) as above average, and his slider (144) as elite. King also tunnels his pitches well, pairing the slider and four-seamer together as well as the sinker and sweeper.

 

King boasts an impressive arsenal, but does he locate his pitches effectively? In short, yes. In 2023, his 22% K-BB% put him in the 90th percentile among pitchers who threw at least 50 innings, and his 65% first strike rate was in the 80th. Ruben Niebla, the Padres pitching coach, had previously revitalized Seth Lugo’s career. What did he do for Michael King this year? 

In 2024, King threw his changeup more frequently, limiting opposing batters to a paltry .207/.271/.286 slash line thanks to a 36.2% whiff rate. Despite a rocky start, he looked like a Cy Young candidate, finishing the season with a 2.95 ERA, 75 ERA-, and 2.3 fWAR over 173 2/3 innings. Starting on June 29, King allowed fewer than three runs in 13 of his 14 starts. King’s dominance in the postseason further cemented his role as a starting pitcher. In his first playoff appearance, he shut down Atlant, throwing seven scoreless innings and recording 12 strikeouts.

Potential Reliever-to-Starter Candidates 

 

Orion Kerkering, RHP Philadelphia Phillies

Before being drafted by the Phillies in 2022, Orion Kerkering was a starting pitcher at the University of South Florida. He filled in as a closer in his final year of college. Kerkering posted a 2.29 ERA, 2.42 FIP, and 28.8% strikeout rate over 63 innings this year. His 102 Location+, 45.% zone rate, and 22.2 K-BB% indicate that possesses solid control. He also has a lot of swing-and-miss stuff in his arsenal.  

Pitch	Usage%	Stuff+	Whiff%	K%	Opp BA
Sweeper	55.7	143	31.1	27.4	.226
Four-Seamer	29.2	114	27.2	35.3	.224
Sinker	15.2	99	16	24.1	.213

Kerkering only started two games as an opener for the Phillies this year. The Phillies starting rotation ranked fourth in the league in innings pitched 903.0. Since their starting pitchers went deep into the game, their bullpen pitchers usually threw one inning or less. However, the back end of the Phillies rotation crumbled apart after the All-Star break. Across 100.4 innings, Ranger Suárez, Tyler Phillips, Taijuan Walker, and Koby Allard recorded a combined 7.46 ERA, and 6.39 FIP. It’s evident the Phillies could use a reliever who can handle bulk innings to bolster their bullpen depth. Kerkering is a good candidate and this would help him get acclimated to longer appearances before the full transition to a starter’s workload.

Mason Miller, RHP Oakland Athletics

 

Mason Miller debuted as a starter for the A’s last season. He made six starts and looked sharp, running a 3.70 ERA and 26.7% strikeout rate over 24 1/3 innings pitches. He was moved to the bullpen after a 60-day IL stint due to a strained UCL. Miller garnered national attention as the A’s primary closer this year. He pitched a couple of multi-inning outings, running a 2.49 ERA, a crazy 41.8 strikeout rate, and a 0.88 WHIP. Miller is often referred to as a pitcher who possesses “starter command with closer stuff”. His 100 Location+, 42.9 Zone%, and 33.3 K-BB% support this characterization. He has an authoritative arsenal to build upon but needs to add a fourth pitch if he moves back to the starting rotation. 

Pitch	Usage%	Stuff+	Whiff%	K%	Opp BA
Four-Seamer	63.1	137	37.1	37.3	.178
Slider	35	160	47.2	51.1	.140
Changeup	1.7	107	44.4	33.3	.000

Miller is an elite closer. He’s only 26 years old and has a long career ahead of him, though the UCL sprain likely means that Tommy John surgery is almost certainly in his fut

ure. With Kenley Jansen, Chris Martin, and Tyler O'Neill entering free agency, the Red Sox need bullpen depth and a right-handed power bat. Miller and outfielder Brent Rooker were frequently mentioned in trade rumors around the deadline. Mason Miller is an elite closer and Brent Rooker put up 5.1 WAR with a 164 wRC+ in 2024. He would represent an immense upgrade from Tyler O’Neill. Both players would help fill the most significant holes in Boston's roster. The price would be steep, but it wouldn’t hurt the Red Sox to call the A’s and see if they work out a trade package. 

The Red Sox are not unfamiliar with moving relievers to the starting rotation. Kutter Crawford and Garrett Whitlock transitioned with mixed results. Whitlock’s career has unfortunately been marred by injuries. Crawford surrendered the most home runs in the league this year, but still managed to post a solid 2.6 WAR. During his end-of-season press conference, Alex Cora told reporters that the team wanted players to focus on being better prepared to face a full season's worth of games, mentioning the Cleveland bullpen as a model to emulate.

Greg Weissert, Josh Winckowski, Brennan Bernardino, Cam Booser, Zack Kelly, and Justin Slaten are expected to resume their bullpen roles next year. Liam Hendriks will return from the injured list, but there’s no guarantee he’ll be back to his All-Star form. Garrett Whitlock is another potential bullpen option. Slaten stands out as the best reliever-to-starter candidate amongst the Red Sox bullpen pitchers. He has a history of being a starter, opening games in college and the minors. Slaten pitched several multi-inning outings this year. With a 21.9 K-BB%, he boasts the command and the effective four-pitch arsenal of a starting pitcher. 

Pitch	Usage%	Stuff+	Whiff%	K%	Opp BA
Cutter	37.7	113	24.1	15	.247
Four-Seamer	29.2	91	26.6	31	.151
Sweeper	24.7	115	35.6	27.3	.279
Curveball	8.4	158	42.3	50	.150

Given the lack of reliable bullpen depth, it’s unlikely Sox fans will see Slaten slotted in the rotation next year. However, it’s something to keep in mind and look for in the future. Baseball is a game of adjustments. Pitchers are increasingly relying on analytics and forward-thinking training techniques to refine their repertoires and help them transition into new roles. Lugo added a slurve and sweeper to his pitch mix. Manaea trained at Driveline in the offseason to augment his fastball’s velocity and then made even more dramatic in-season changes. King made adjustments to his slider and worked on controlling his changeup. The success of these pitchers, along with Garrett Crochet and (in 2023) Cole Ragans, highlights a growing trend of former relievers excelling after transitioning into the starting rotation. Utility pitchers could soon become as common as utility position players, offering teams flexibility and depth in managing their pitching staffs. Pitchers' roles continue to evolve with the rise of openers and multi-inning relief pitchers. As injury issues continue to plague starting pitchers and the demand for them increases, the reliever-to-starter trend is likely to become more prominent in the upcoming seasons.

 

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