The Chicago White Sox are moving trade deadline acquisition Luis Castillo to the bullpen following his disappointing start with the team. He has a 7.76 ERA and 1.79 WHIP over 29 innings with Chicago.

He also has a 25-to-11 strikeout-to-walk ratio. The three-time All-Star went 11-8 with a 3.54 ERA for the Seattle Mariners in 2025, and his track record helped prompt the White Sox to acquire him at the deadline. 

Castillo has not provided the production the White Sox hoped for when they acquired him. David Sandlin is expected to handle bulk innings as Chicago begins its final 10 games of the regular season.

The White Sox slipped into second place in the AL Central on Wednesday as they continued their push for a postseason berth.

White Sox’s Luis Castillo trade looking worse after bullpen shift

Castillo’s struggles began earlier this season with the Mariners, when opponents batted .270 against him over 20 starts.

His performance continued to decline after the trade, with opponents batting .328 against him. Castillo’s struggles with both clubs have left him with a 5.67 ERA this season, prompting Chicago to move him out of the rotation.

Castillo already had made a bulk-relief appearance against the St. Louis Cardinals on Sept. 12, when he allowed three earned runs in 2 2/3 innings. He also surrendered six earned runs against the Houston Astros on Sept. 3.