Castillo was placed on the 10-day disabled list retroactive to Friday with a right hamstring strain.
Castillo last pitched Wednesday and allowed one run with a walk and no hits allowed over two-thirds of an inning. The 22-year-old has been stellar out of the Padres bullpen since he was promoted in early June, recording a 2.84 ERA, 0.79 WHIP, 18 strikeouts and four walks across 12.2 innings.