The Seattle Mariners chipped away at their starting-pitching surplus on Saturday night, as they traded Luis Castillo to the Chicago White Sox for reliever Seranthony Domínguez and a pair of minor leaguers.
The Mariners were widely expected to dangle starting pitchers ahead of Monday’s trade deadline, with Castillo being the worst-performing and highest-paid of the bunch. Emerson Hancock and George Kirby have been floated as possible targets for other teams as well.
Seattle has squeezed six starters into its rotation this season, sometimes via the use of a piggybacking system. Now, Castillo lands in Chicago, where the AL Central-leading White Sox have been the surprise of the league. In addition to Domínguez, the White Sox sent outfielder Nolan Jones and catching prospect Boston Smith to Seattle.
Castillo was expendable for the Mariners, who, in addition to their group of big-league starters, also have top prospect Kade Anderson silencing hitters at Double A. Anderson, the third overall pick in last year’s draft, owns a 1.20 ERA and a .408 opponent OPS. The Athletic’s Keith Law recently ranked him the top prospect in the sport.
Castillo had been a stalwart for the Mariners since they acquired him in a 2022 deadline deal with Cincinnati. In the next three seasons, he went 36-29 with a 3.50 ERA, making at least 30 starts each year and earning the nickname “The Rock” for his consistency and reliability.
But “The Rock” began to crumble last fall in the ALCS, when Castillo pitched just once in a seven-game series loss to the Toronto Blue Jays, who knocked him out in the third inning of his loss in Game 4. This season has been the worst of his career.
Castillo has lasted six innings only five times in 20 appearances — a consequence, at times, of the Mariners’ rotation depth. With five other starters healthy, the Mariners have sometimes tried a “piggy-back” approach, which irked Castillo when he was removed from a start against the Athletics after four scoreless innings.
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Starting with that game, on May 25, through July 21, Castillo was fairly solid, with a 3.95 ERA. In his last start before the deadline, he gave up five runs and 11 hits in five innings.
Castillo’s velocity is down from his later Reds seasons, when he averaged better than 97 mph. But his average fastball this season has measured 95.3 mph, which should be plenty hard enough to succeed with a sinker, slider and changeup.
This deal marks the third consecutive year in which Domínguez has been traded ahead of the deadline. In 2024, the Phillies sent him to the Orioles. Last summer, Baltimore traded him to the Blue Jays and he wound up making 12 appearances during Toronto’s run to the World Series. He signed a two-year, $20 million deal with the White Sox in January and was pitching in high-leverage situations for an AL Central-leading Chicago team that has been the surprise of the league. Domínguez has logged a 4.10 ERA in 42 appearances this season, with 45 strikeouts in 37 innings.
It’s the second time Jones, once a top prospect in the Guardians’ organization, has been traded this season, and the fourth time in his career. After a couple of subpar seasons — one in Colorado, one in Cleveland — the Guardians sent him to the White Sox in June for $250,000 in international bonus pool money. Overall, Jones has compiled a .257/.362/.445 slash line in his Triple-A stops this season.
Smith, meanwhile, has had the most impressive year of the three players heading to Seattle. The 23-year-old boasts a .291/.448/.589 slash line this season, having jumped two levels in his first year of professional ball, and was batting .306 with eight homers in 22 games with Double-A Birmingham. He was a sixth-round pick out of Wright State last summer, and The Athletic’s Keith Law wrote last week that he “looks like he’ll be a backup catcher with power, and he has a chance to become a regular thanks to the athleticism behind the plate.”
The Athletic has live coverage of the 2026 MLB Trade Deadline.
