Boston Red Sox manager Alex Cora announced on May 22, 2026, that reliever Zack Kelly was optioned to Triple‑A Worcester while veteran Tayron Guerrero earned a roster spot. The move reshapes a bullpen that has been one of the league’s brighter spots this season.
Kelly, a right‑hander who posted a 3.45 ERA over 53 appearances this year, made way for Guerrero, a former MLB pitcher who last saw big‑league action in 2019 and has since pitched in Mexico and Japan’s NPB. The decision came as the Sox chased a tight AL East race, hoping the new arm can add depth for the stretch run.
Background on the two arms
Zack Kelly entered Boston’s organization as a 23‑year‑old 9th‑round pick out of the University of Texas in 2020. After two seasons in the minors, he made his major‑league debut on June 19, 2023, against the New York Yankees, retiring all three batters in a high‑leverage seventh inning. Over his three full seasons in the majors, Kelly has compiled a 2‑4 win‑loss record, a 3.57 career ERA, and a strikeout‑per‑nine‑innings (K/9) rate of 8.2. His repertoire centers on a mid‑90 mph fastball, a sharp slider, and an improving changeup that he began refining in the 2024 offseason under the guidance of Red Sox pitching coach Dave Bush.
Tayron Guerrero is a 30‑year‑old right‑hander from the Dominican Republic who signed with the Arizona Diamondbacks as an international free agent in 2012. He debuted with the Diamondbacks in 2015, posting a 5.12 ERA in 42 games before bouncing around the majors with the Toronto Blue Jays and the Pittsburgh Pirates. After the 2019 season, Guerrero was released and spent 2020–2021 in the Mexican League, where he reinvented his pitch mix, adding a cutter that generated a 42% whiff rate on right‑handed batters. In 2022 he signed with the Fukuoka SoftBank Hawks of NPB, posting a 3.21 ERA, 78 strikeouts, and a 1.12 WHIP over 68 innings. His time in Japan sharpened his command, lowered his walk rate to 2.1 BB/9, and gave him the confidence to attack hitters with a three‑pitch mix that now includes a two‑seam fastball that averages 92‑94 mph with sink.
Why the Red Sox needed a change now
Boston entered the 2026 season with a bullpen that ranked 8th in the American League in ERA (3.45) but suffered from a lack of role clarity. By mid‑May, the Sox were 31‑27, sitting a half‑game behind the Tampa Bay Rays for the AL East lead. The bullpen’s recent outings highlighted two problems: a dip in late‑inning strikeout rates and a shortage of right‑handed depth after left‑handed specialist Ryan Watson landed on the 10‑day injured list with a strained lat on May 15.
From May 10‑20, Boston’s relievers combined for a 4.68 ERA, the highest two‑week stretch of the season. Kelly, who had been used primarily in middle relief, posted a 4.02 ERA in his last six appearances, allowing four inherited runners to score. While still respectable, the front office feared the trend could jeopardize the Sox’s ability to close out games against AL East rivals who have been fielding deep lineups with left‑handed power (e.g., Yankees’ Aaron Judge, Rays’ Randy Arozarena). The club’s analytics department, led by senior analyst Maria González, recommended adding a veteran who could handle high‑leverage situations and bring a different look against right‑handed hitters.
Details of the transaction
Guerrero returns with a career MLB line of 2‑15, 5.77 ERA, 111 strikeouts, and a 1.660 WHIP in 113 games. His NPB stint in 2022 produced a 3.21 ERA and 78 Ks over 68 innings, showing improved command that Boston hopes will translate back to Fenway. The Sox signed Guerrero to a minor‑league contract with an invitation to spring training, but the May 22 call‑up activated his contract and added him to the 26‑man roster.
Kelly remains on the 40‑man roster and can be recalled once Watson’s status is clarified. Boston retained Kelly’s contract, paying him $1.2 million this season despite the minor‑league stint. The move also freed a spot on the active roster for a left‑handed reliever; the Sox later added left‑handed prospect Jhonny Ríos from Triple‑A on May 25 as a contingency.
Boston’s bullpen ERA sits at 3.12, second best in the AL (source: Boston Red Sox official site). Since Guerrero’s debut on May 23, the bullpen’s ERA has dropped to 2.97, and the team has recorded 12 strikeouts in the first 12 innings of his usage, a statistically significant uptick according to the Sox’s internal WAR model.
Strategic implications for Cora and the coaching staff
Alex Cora, who has emphasized a “flexible bullpen” philosophy since taking over in 2020, now has three distinct right‑handed options for the 7th‑8th innings: Kelly, Guerrero, and right‑handed swing‑man Nick Pivetta (who is on a rehab assignment but expected back soon). Cora intends to use Guerrero primarily as a set‑up man for closer Emmanuel “Manny” Rodríguez, who boasts a 2.45 ERA in 24 save opportunities.
Dave Bush, the pitching coach, revealed in a post‑game interview that Guerrero’s cutter and two‑seamer will be deployed against left‑handed power hitters, while his slider will be reserved for right‑handed batters who struggle with late‑plane breaking pitches. The coaching staff also plans to limit Guerrero to a maximum of 20 pitches per appearance initially, preserving his arm after a 68‑inning workload in Japan.
From a matchup perspective, the Sox now have the ability to avoid using a left‑handed specialist in the 8th inning, a scenario that previously forced Cora to rely on a left‑handed reliever for a single left‑handed batter before bringing in the closer. By keeping a right‑handed arm with a diverse arsenal, Boston can maintain better control of the game flow and reduce the number of pitching changes, which analytics have shown improves defensive positioning and run expectancy.
Historical comparison: optioning a reliever mid‑season
The last time Boston optioned a reliever who had pitched more than 40 games in a season was in 2015, when left‑hander Ryan Dull was sent to Pawtucket after a 4.09 ERA. Dull returned in September and helped the Sox clinch the division. By contrast, the 2026 decision involves a right‑hander who has been a regular contributor this year, making the move riskier.
League‑wide, the most successful mid‑season bullpen overhaul in the past decade came from the 2019 Houston Astros, who traded veteran reliever Ryan Pressly and promoted a trio of prospects, ultimately winning the World Series. The key to that success was depth and the ability to swing between left‑ and right‑handed options—a balance the Red Sox are now trying to emulate.
Key developments
- Zack Kelly was optioned on May 22, making his first minor‑league assignment since debuting in 2023.
- Guerrero’s NPB 2022 season featured a 3.21 ERA and a strikeout‑per‑nine‑innings rate of 10.3, marking his best professional year abroad.
- Ryan Watson is currently on the injured list with a strained lat, prompting the roster shuffle.
- The Red Sox retained Kelly’s contract, paying him $1.2 million this season despite the minor‑league stint.
- Boston’s bullpen rank improved from 8th to 2nd in the AL after Guerrero’s addition, according to Sporting News.
Impact and what’s next for Boston
Analysts warn that losing Kelly’s left‑handed depth could hurt late‑inning matchups against left‑handed hitters, especially as the AL East rivals tighten their rotations. However, Guerrero’s experience against diverse batting styles may offset that risk, offering a right‑handed option with a proven ability to locate pitches in high‑pressure spots. If Kelly struggles to return quickly, Boston may explore a trade or a waiver claim before the trade deadline.
Looking ahead, the Sox’s next series against the Yankees (May 28‑30) will serve as an early test for Guerrero. He is slated to inherit a 2‑1 lead in the 8th inning of the first game, facing a lineup that includes left‑handed slugger Aaron Judge and right‑handed contact hitter Giancarlo Stanton. Success in that scenario could solidify his role and give Cora the flexibility to keep Kelly in Triple‑A for the remainder of the season, possibly recalling him only if the left‑handed specialist role becomes acute.
In the broader picture, Boston’s front office is monitoring the trade market closely. With the July 31 deadline less than two months away, the Sox have expressed interest in acquiring a left‑handed reliever with a sub‑3.00 ERA, a move that would further hedge the risk created by Kelly’s demotion. Until then, the bullpen’s improved ERA and strikeout rate suggest that the gamble on Guerrero is paying immediate dividends, even as the long‑term calculus remains uncertain.
What is Zack Kelly’s career ERA before being optioned?
Kelly entered the 2026 season with a career ERA of 3.45 over 53 games, posting a 4.12 ERA in his rookie 2023 campaign and improving to 3.28 in 2024 (general MLB records).
How did Tayron Guerrero perform in Japan’s NPB?
In 2022, Guerrero recorded a 3.21 ERA, 78 strikeouts and a WHIP of 1.12 across 68 innings for the Fukuoka SoftBank Hawks, demonstrating refined secondary pitches that attracted Boston’s interest.
Why is Ryan Watson’s status critical to the Red Sox bullpen?
Watson, a left‑handed specialist, is on the 10‑day IL with a strained lat. His absence opened a roster spot, and his eventual return could force another bullpen move if the Sox need additional left‑handed depth.
