The Twins designated Castillo for assignment Wednesday.
He'll give up his spot on the 40-man roster and 26-man active roster to right-hander Brock Stewart (shoulder), who was reinstated from the 60-day injured list prior to Wednesday's game against the Phillies. After getting a call-up from Triple-A St. Paul on July 14, Castillo appeared in just four games while going 2-for-6 with a double, two walks, three runs and two RBI.