The White Sox acquired Castillo from the Mariners on Saturday in exchange for right-hander Seranthony Dominguez, outfielder Nolan Jones and catcher Boston Smith, Jeff Passan of ESPN.com reports.
The 31-year-old righty will head to Chicago and bolster a rotation that's received excellent results from Davis Martin and Sean Burke but uneven production from Anthony Kay, Noah Schultz and Erick Fedde at the other three spots. While the three-time All-Star easily becomes the White Sox's most battle-tested starter, Castillo has endured a career-worst 2026 campaign. After submitting a 3.54 ERA and 1.18 WHIP in 32 starts for Seattle a season ago, Castillo turned in a 5.06 ERA and 1.39 WHIP in 20 appearances (17 starts) in 2026 prior to being dealt. Castillo showed signs of righting the ship in May and June, but he went 0-3 with a 5.48 ERA and 1.52 WHIP while serving up seven home runs in 23 innings over four July outings. He last pitched Tuesday, so Castillo could be ready to make his White Sox debut during the team's series in Boston early next week.