Manager Will Venable said Thursday that Castillo has been moved to the bullpen, Jesse Rogers of ESPN.com reports.
Castillo's Sept. 12 was in a bulk-relief role, which did not go well. He surrendered three runs on fours hits and three walks across 2.2 innings against the Cardinals. It came after what would be his final start of the season Sept. 3, when he allowed six earned runs across four innings in Houston. The right-hander was in line to start Friday versus the Tigers, but it will be David Sandlin taking the mound to start. As the White Sox are in position for the playoffs, they likely prefer Castillo out of the bullpen than as a postseason starter.