Castillo is slated to pitch in bulk relief behind opening pitcher Sean Newcomb during Saturday's game against the Cardinals, James Fegan of the Chicago Sun-Times reports.
The top of the St. Louis lineup typically features two left-handed hitters (Bryan Torres and Alec Burleson) against right-handed pitchers, so the White Sox will have the left-handed Newcomb work the first inning or two in order for Castillo to draw some more favorable matchups when he eventually enters the game. Castillo will be pitching in relief for the first time since being acquired by the White Sox at the trade deadline, with the role change coming after he logged a 7.52 ERA, 1.71 WHIP and 23:8 K:BB in 26.1 innings through his first six starts with the club.