The White Sox and Mariners have agreed to a trade that will send right-hander Luis Castillo to Chicago in exchange for right-hander Seranthony Domínguez, outfielder Nolan Jones, and catching prospect Boston Smith. Both teams have officially announced the trade, which was initially reported by ESPN’s Jeff Passan.

ESPN’s Alden González adds that the White Sox will take on all of the money remaining on Castillo’s contract. The veteran righty has about $7.4MM remaining on his 2026 salary plus $22.75MM in 2027. Castillo’s contract also includes a $25MM vesting option for 2028 that triggers if Castillo throws 180 innings in 2027 and is healthy going into the following year.

Dominguez’s own contract provides some salary offset, as the reliever is owed about $15.2MM ($3.2MM in remaining in 2026 salary, $10MM in 2027, and a $2MM buyout of a 2028 mutual option) over the remainder of the two-year deal he signed with the White Sox in January.

It’s a major swing for a Chicago squad known to be looking for veteran starting pitching help. The club lost a tight game to the Rays on Saturday, but still maintains a slight lead in the AL Central. Breakout arms Davis Martin and Sean Burke currently headline the White Sox staff. Free agent addition Anthony Kay has offered decent results. The club has filled out the rest of the rotation with Erick Fedde behind an opener and struggling rookie Noah Schultz.

The 33-year-old Castillo is battling through his worst year as a big leaguer. He’s posted a 5.06 ERA in 20 appearances. Despite the struggles, Seattle made an effort to keep him in a crowded rotation even after Bryce Miller returned from injury, as Castillo operated behind an opener for three games in the first non-starting appearances of his decade in the majors. Seattle now clears its rotation glut, while Chicago picks up an experienced starter with six playoff appearances under his belt.

This is Castillo’s second time getting moved at the deadline. He was shipped from Cincinnati to Seattle in 2022 for infielders Noelvi Marte and Edwin Arroyo and right-handers Levi Stoudt and Andrew Moore. The Mariners parted with a big prospect in Marte, but they were rewarded handsomely on their end. Castillo was a sturdy member of the rotation up until this season. He delivered a mid-3.00s ERA and 30+ starts in each of the past three years.

Castillo’s arsenal began to slip last season. He averaged a career-low 95.0 mph on his four-seamer. The righty had a Stuff+ below 100 for the first time in the majors. While his fastball has ticked up to 95.3 mph, Castillo is down to a career-low 97 Stuff+. His 19.4% strikeout rate and 10.9% swinging-strike rate are the worst marks of his MLB tenure.

With these numbers in mind, it counts as a win for the M’s that they were able to get Castillo mostly off their books. The righty’s hefty contract is the primary reason why Castillo has been the Mariners starter considered most available in trade talks over the last couple of years, as the M’s operate within a fairly mid-level budget. While other trade suitors have naturally asked more about Seattle’s younger and more controllable arms (George Kirby, Logan Gilbert, Bryan Woo, and Miller), moving Castillo was seem as the Mariners’ preferred method of dealing from their rotation surplus and freeing up payroll space.

Emerson Hancock was also viewed as a pitcher the Mariners were open to moving, plus reports emerged today that the M’s were at least listening to offers involving Kirby. Interestingly, the Athletic’s Ken Rosenthal writes that Seattle is still “open to moving Kirby” even in the wake of the Castillo trade. Since Ryan Sloan and Kade Anderson are nearing big league readiness, the M’s could install one of those top prospects into Kirby’s rotation spot, while Kirby is dealt to address another need (probably offense). Seattle can now theoretically expand its search for bats now that the team has a bit more payroll room to operate, both now and at least in 2027.

From Chicago’s perspective, the Sox have now landed the veteran arm they’ve been seeking as reinforcement for its inexperienced rotation. Such pitchers as Reid Detmers, Trevor Rogers, Clay Holmes, Shane Bieber, and Dustin May were linked to the White Sox, and it is possible the Pale Hose could seek out another arm as further rotation help.

Given that Dominguez’s deal is going the other way, the Sox are adding a little over $15MM in extra payroll to accommodate Castillo. The vesting option is a significant future expenditure, but if Castillo is healthy and effective enough to log 170 innings next year, that’s a “problem” the White Sox might welcome in retaining the righty as a veteran centerpiece.

The obvious risk is that Castillo’s struggles could be related to a true decline rather than just some growing pains in settling into his piggyback starter role, but that’s a risk the Sox are willing to take in their push for a playoff spot. Turning back to that 2022 deal, Castillo’s clubhouse leadership and quality work on the mound were cited as major reasons the Mariners made the playoffs for the first time since 2001, so Chicago might be counting on a repeat with this latest trade.

Ironically, the idea of the White Sox adding Castillo at the deadline might’ve been plausible at the start of the year, if one imagined a pure salary dump scenario where Seattle attached some prospects as a sweetener to move Castillo’s contract. We instead find ourselves in a world where the seemingly rebuilding White Sox are leading the AL Central and are current in position for a first-round bye, while the Mariners (last year’s ALCS finalists) are 54-58 and trying to stay close to the AL West and wild card races.

Signed to act as Chicago’s closer, Dominguez lost his ninth-inning role after posting a 4.10 ERA, 27.8% strikeout rate, and a very high 14.2% walk rate in his first 37 1/3 innings of the season. For what it’s worth, Dominguez’s numbers have improved since he has been moved to lower-leverage assignments, so he could certainly provide some value to the Mariners beyond just salary offset.

Control and home runs have been persistent issues for Dominguez over his career, but at his best, the right-hander has been a very solid reliever, even entrusted with some closing opportunities before his arrival on the South Side. Pitching in Seattle could help Dominguez keep the ball in the park, and he’ll slot into a middle relief role behind M’s closer Andres Munoz.

Jones gives the Mariners some outfield depth in the wake of Luke Raley‘s IL placement earlier today. Jones is best known for his past status as a top-100 prospect during his time in Cleveland’s farm system, and the seeming breakout year he enjoyed in 2023 after he was traded to the Rockies. A .401 BABIP was a big outlier in that 2023 season, however, and things fell back to earth for Jones in 2024-25 when he hit only .218/.307/.311 over 700 plate appearances in the majors.

The Guardians re-acquired Jones from Colorado in March 2025 but then parted ways with the outfielder again by trading him to the White Sox in June. Jones hasn’t yet seen any big league action in 2026, but he has a solid .257/.362/.445 slash line and 14 home runs over 382 PA at the Triple-A level. At age 28, Jones might still have some late-bloomer potential with another change of scenery, but it’s probably more likely that he will settle into a depth role.

The 23-year-old Smith was a sixth-round pick for the Nationals in the 2025 draft, and barely over a year into his pro career, Smith has already been involved in two prominent trades. Washington dealt Smith to the White Sox in March for Curtis Mead, in a deal that worked out swimmingly well for the Nats — Mead had a breakout year and was just flipped to the Red Sox for Connelly Early.

Smith didn’t make his pro debut until this year, and he already played at three different levels while hitting .291/.448/.589 with 22 home runs over 346 combined PA for Chicago’s A-ball, high-A, and Double-A affiliates.  Smith has struck out 96 times in those 346 PA, but his high walk rate and cartoonish power numbers can excuse some swing-and-miss.

Adding to Smith’s skillset is his ability behind the plate, though MLB Pipeline (who rank Smith 14th in Seattle’s farm system) views him as a work in progress as a catcher. Smith has also gotten significant time in left field this year, so should he make it to the majors, he could be more of a hybrid backup than a true catcher. This could help him stick in the lineup since Cal Raleigh has the Mariners’ catcher spot on lockdown, plus the M’s have another well-regarded catching prospect in Luke Stevenson.

Inset photo courtesy of Kevin Ng – Imagn Images