Castillo was optioned to Triple-A Buffalo on Saturday.
The 23-year-old made a pair of starts ahead of the All-Star break but moved to the bullpen over the last week since Yusei Kikuchi is now back to full health. Castillo struck out one in two scoreless innings over his last two relief appearances but will head to the minors after Trent Thornton was recalled Saturday. Castillo will likely be used as a starter in the minors, Ben Nicholson-Smith of Sportsnet.ca reports.