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Relief pitcher is the most volatile position group in baseball. Contracts are smaller, arms are fungible, and transitions from reliever to starter and back are becoming more common. The Red Sox had eight relievers on their Opening Day roster in 2023. Only one of them had been on the Opening Day roster in 2022, and only three of them would be on the Opening Day roster in 2024. Last season, the bullpen’s 4.39 ERA and 4.11 FIP ranked 24th in baseball, while its 613 innings pitched ranked 10th. With newly-acquired ace Garrett Crochet still working his way toward a normal starter’s workload and Lucas Giolito ramping up after missing the entirety of last season, the team will likely need both more quantity and more quality out of the bullpen if it plans on contending in 2025.
With that in mind, this will be the first of a five-part series examining 15 Red Sox relievers from 2024. The goal is to understand where they finished the season in order to get a sense of what the 2025 ‘pen will look like. We’ll start with three of the team’s 2024 bullpen stalwarts.
Greg Weissert
Greg Weissert led all Red Sox relievers with 62 appearances and 63 1/3 innings in 2024. The 29-year-old right-hander is still on a rookie contract and won’t be arbitration eligible until 2026, so his performance and durability during the 2024 season are encouraging signs for the future. Weissert’s 3.13 ERA was second-best among Red Sox relievers, modeling some semblance of consistency out of the pen. However, Weissert’s peripherals indicated some real reasons for concern. This was his first full season in the majors and his 3.76 FIP was more than half a run higher than his ERA, which indicates that he might have benefitted from some good luck. Weissert earned a whiff on just 20% of his pitches, which put him in the 10th percentile. Pitchers who miss bats so infrequently need to be great a inducing weak contact or keeping the ball on the ground, and Weissert didn’t excel in either category. He’ll need to improve in at least one of those three areas if he hopes to keep his success going.
Justin Slaten
Objectively, Justin Slaten was the Sox’ best reliever this year. In his first taste of the majors, he led the bullpen with 1.5 fWAR and a 2.93 ERA, and his 55.1 innings were the second only to Weissert. He was also able pitch more than an inning, racking up his 55 1/3 IP in just 44 appearances. Slaten featured one of the more impressive pitches in baseball this year. Among pitchers who threw at least 30 innings in 2024, his curveball ranked first in Stuff+ with an absurd 158 score. Slaten did just about everything well. His 35% chase rate was one of the best in all of baseball, and he was excellent at missing bats, avoiding hard contact, and keeping the ball on the ground. His 2.61 FIP was even better than his ERA (likely because he was inducing groundballs in front of Boston’s lackluster infield defense). His 4% barrel rate put him in the 96th percentile, just one spot behind Cy Young candidate Emmanuel Clase.
The 27-year-old Slaten won’t enter arbitration until 2026 and he won’t be a free agent until 2029. As with Weissert, Slaten seems like an opening day must for the Red Sox and his strong peripheral stats suggest he could be just as good in 2025.
Chris Martin
In his second year with the Sox, Chris Martin saw his ERA balloon from 1.05 to 3.45. However, he was one of just five Red Sox relievers with a sub-3.50 ERA in more than 10 innings pitched. The good news? Martin’s FIP only rose by .34 and his expected FIP was better in 2024 than in 2023. Furthermore, Martin maintained many of the traits that contributed to his incredible ERA in 2023.
Martin walked just 0.6 batters per nine innings, the least among all 402 pitchers 474 pitchers who threw at least 30 innings. He allowed just five barrels all season, and Stuff+ saw his splitter as elite, giving it an impressive 126 score. So why did Martin’s ERA rise? The problem was twofold for him in 2024. In 2023 he outperformed many of his expected metrics. His 3.18 expected ERA was much higher than his actual ERA. It was almost guaranteed he would regress somewhat this past season. Additionally, in 2024 Martin was extremely unlucky. he posted a .353 BABIP which was eighth-highest among relievers to throw more than 30 innings.
Martin is a free agent and is entering his age-39 season. He’s indicated that 2025 will be his last season, and he’s rumored to be interested in pitching closer to his home in Texas. However, the the 6’8” monster of a man clearly had plenty in the tank in 2024, and the Red Sox could definitely use him. No matter where the righty ends up, expect elite command and expect his underlying numbers to translate to more success than they did in 2024.
This first iteration of the series is pretty cut and dry. Weissert and Slaten are excellent candidates to record serious innings for the squad next year. Martin should be a serious option for the Red Sox to sign in free agency. The next part of the series will dive further into the meat of the pen and will include Chase Anderson, Brennan Bernardino and Zack Kelly.







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