The Twins designated Castillo for assignment Wednesday, Do-Hyoung Park of MLB.com reports.
The Twins needed to free up a spot on their 40-man roster for the addition of right-hander Jay Jackson to the bullpen. Castillo had just been sent down to Triple-A St. Paul on Tuesday following a brief stint with the big club in which he allowed two runs with a 3:7 K:BB over seven innings. If Castillo clears waivers, he will have the ability to elect free agency if he chooses to do so.