Breaking Down the Boston Bullpen: Part 5

Every day this week, we've looked at the relievers who got the Red Sox through the 2024 season. We've examined the best, the bulkiest, and the worst, and those whose appearances had the power to make or break a game. Today, we'll focus on three pitchers who filled in gaps for the Sox in 2024.

Brad Keller
Brad Keller signed as a free agent in May and threw 24 2/3 innings for the Sox. A starter for most of his career, Keller moved to the pen after only pitching in 11 games for the Royals in 2023. He made two starts for the White Sox before he was released and signed in Boston four days later, making 11 big-league relief appearances between May 27 and August 26. Keller's 5.44 ERA was the worst of his career, and he flipped back and forth between Worcester and Boston over those three months. The righty had a negative WAR and saw his typically high groundball rate fall. He ate innings, but wasn't able to provide much more value than that. Keller is entering his age 29 season and is currently a free agent after the Red Sox outrighted him in August and he elected free agency. If he returns to the big leagues this season, it will likely be on another minor league contract.

Joely Rodríguez
Joely Rodríguez played the role of the lefty fill-in for the Sox last year. He threw 13 2/3 innings as he moved back and forth between the minors and the majors. Rodríguez ran a 4.28 ERA from 2020 to 2022, but his ERA ballooned to 6.55 in 2023. If the Red Sox were hoping for a bounce back season, they ended up disappointed. He ran a 5.93 ERA, and many of his peripherals trended south. His 20% whiff rate was the worst of his career, and his strikeout rate plummeted to just below 19%, a drop-off of nearly 10 percentage points from 2023.

However, if you dig deeper, there were a few extremely encouraging signs, as evidenced by Rodríguez's 3.03 xFIP, the third-best on the team. Rodríguez was excellent at avoiding hard contact and keeping the ball on the ground during his short time in Boston, but the results still weren't there. Pitch models weren't impressed by his repertoire, but it's worth noting that they're much more geared to appreciate pitches that rack up swings and misses, and that's not Rodríguez's game. He featured an interesting pitch mix. He threw a sinker, changeup, and sweeper, with extreme East-West movement. There were some clear issues, some of them fixable. Rodríguez likely can't do much to increase the speed of his 90-mph sinker, but he could have done a better job of locating it lower in the zone. He also ran an excellent chase rate, which should have allowed him to pitch outside the zone more often; instead, he ran one of the highest zone rates in baseball. His sinker and changeup have extremely similar movement profiles, but they have just a 4.5 mph velocity differential, which is extremely low. If he can find a way to keep his sinker down and to get a little bit more separation between it and his changeup, he could make for an interesting reclamation project.

Rodriguez was outrighted in August, and elected free agency rather than returning to the minors. The 31-year-old is currently pitching in the Dominican Winter League for Gigantes del Cibao.

Luis Guerrero
Rookie Luis Guerrero was excellent in an extremely small sample in 2024. He allowed just one earned run over 10 innings during his major-league debut. The 23-year-old had a 3.31 ERA in Triple A before being called up to the bigs. Guerrero's fastball averaged 97.5 mph, and opponents hit just .100 against it. Guerrero is very much a prospect, and during his time in Boston, he showcased his potential. He features serious stuff and velocity, and when he's able to control it, he's extremely hard to hit. That stuff allowed him to cruise through the lower minors, but he's now reached his level, and will have to learn to command his pitches in order to succeed in Boston. The first 10 innings were certainly encouraging. Guerrero will likely pitch for the Sox in 2024, and may have a solid chance of starting the season with the big club. His role at the beginning of the season will depend on his performance during spring training, and his ultimate fate will likely depend on his walk rate.

All three of these pitchers made limited contributions to the Sox in 2024. However, their career paths are drastically different. Keller and Rodriguez will be looking for minor-league deals, while Guerrero still has his rookie eligibility and could make a huge splash if Boston's pitching development team can help him figure things out.