The Royals recalled Castillo from Triple-A Omaha on Sunday.
Castillo was sent to Omaha in late May and has since posted a 3.52 ERA and 0.89 WHIP to go with a 28:5 K:BB through 38.1 innings. He's been used primarily as a starter in the minors but figures to serve as a multi-inning reliever while with the Royals. Collin Snider was optioned to Triple-A in a corresponding move.