The Mariners traded Castillo to the White Sox on Saturday in exchange for Seranthony Dominguez, Nolan Jones and Boston Smith, Jeff Passan of ESPN.com reports.
The 31-year-old will head to Chicago and give the White Sox another right-hander out of the rotation. Castillo struggled across his four starts in July, going 0-3 with a 5.48 ERA and 1.52 WHIP over 23 innings. That said, the White Sox are hoping to get the version of Castillo that recorded a 3.65 ERA or lower in each of the four seasons prior to 2026, including a 2.99 ERA over 150.1 innings in 2022, when he pitched for both the Reds and Mariners.