The Royals recalled Castillo from Triple-A Omaha and will have him start Sunday's game in Detroit.
Reliever Wyatt Mills was optioned to Triple-A to clear room on the 28-man active roster for Castillo, who is set to make his second appearance for Kansas City since being acquired from the Blue Jays on Aug. 2. Castillo will be taking over the rotation spot that belongs to Zack Greinke (forearm), who remains on the injured list without a clear timeline to be activated. In his previous start with the Royals on Aug. 18, Castillo took a loss to Tampa Bay after recording three strikeouts and allowing one earned run on three hits and one walk over five frames.