Our first big deal of the 2026 MLB trade deadline is in the books. The Seattle Mariners are sending veteran starter Luis Castillo to the Chicago White Sox for reliever Seranthony Domínguez, outfielder Nolan Jones, and prospect Boston Smith.
Castillo, 33, has struggled for the Mariners this season. Once a big part of their rotation, his ERA has ballooned to 5.06 this season. The Mariners experimented with an opener in front of him, but it didn’t unlock much. Castillo still has good command and a solid chase rate, but he’s not the strikeout pitcher he used to be and is giving up a lot of hard contact. The AL Central-leading White Sox will be hoping that Castillo can find some consistency in their rotation, and he’s likely to replace Noah Schultz or Anthony Kay.
Castillo makes $22.75 million this season, and the White Sox will be on the hook for the roughly $7.2 million remaining this year. He’s under contract for the same amount next year, with a $25 million vesting option in 2028 that triggers if Castillo throws 180+ innings in 2027.
Going back to the Mariners will be a trio of players, headlined by Seranthony Domínguez. Domínguez was a big offseason acquisition for the White Sox, getting a two-year $20 million deal to be the closer on the South Side. He was decent in that role, posting a 3.99 ERA with 12 saves in 43 appearances. Domínguez still throws awfully hard, but is prone to walks and hard contact. He figures to settle into a middle relief role behind Mariners closer nolaAndres Muñoz.
Seattle is on the hook for the remainder of Domínguez’s $8 million salary this year, amounting to roughly $2.6 million. The big reliever is under contract next season for $10 million, and has a $10 million mutual option for 2028 that includes a $2 million buyout.
Minor league outfielder Nolan Jones is also heading to the Mariners. Once a promising player for the Colorado Rockies, Jones regressed heavily and was traded to the Cleveland Guardians, then the White Sox, who optioned him to the minors. In AAA play this season, the 28-year-old Jones has an .807 OPS with 14 home runs and 11 stolen bases.
C/OF prospect Boston Smith is the third player heading to Seattle in the deal. The 15th-ranked prospect in Chicago’s system according to MLB Pipeline, Smith has been crushing in the minors. He owns a 1.037 OPS with 22 home runs across three minor league levels, going from Single-A to High-A to Double-A in the same year. Smith, a 23-year-old, has been a fast riser as a four-year NCAA player. It’s unclear whether Smith is good enough defensively to stay behind the plate, but if the bat keeps developing, Seattle will have to find a spot for him.
On the surface, it’s a strange trade for the White Sox. Their rotation has been a weak spot, but Castillo is clearly on the decline and takes up a huge chunk of their cost-conscious payroll. It also cost them a solid reliever and a good prospect. Perhaps it’s a sign the White Sox are willing to spend more with a playoff berth within their grasp; though, curiously, Castillo is the guy they are willing to splurge on.
For Seattle, the deal makes a lot more sense. Castillo was the most expensive player on Seattle’s roster and easily the weakest point of their deep rotation. The Mariners already have Logan Gilbert, Bryce Miller, George Kirby, Emerson Hancock, and Bryan Woo in the rotation, with Kade Anderson — the best starting pitcher prospect in baseball —knocking on the door. Something had to give. They’ll save almost $20 million over the next season and a half by making the deal, while adding a good reliever to the middle of the bullpen. With the Mariners on the outside of the playoff picture looking in, doing some lateral moves like this while keeping the budget flexible makes a lot of sense.
This trade also has important implications for both teams. If the White Sox are truly buying, things will get awfully interesting. And if the Mariners are deciding to sell, they have the kind of talent that can alter the course of this deadline. Stay tuned.
