Castillo fired a clean inning with three strikeouts while picking up a hold in Thursday's victory over the Cubs.
The 22-year-old has fired 3.2 scoreless innings with two walks and four strikeouts since being activated from the disabled list July 28. The Padres didn't move veteran relievers Kirby Yates or Craig Stammen at the deadline, so Castillo will resume his role as a middle reliever. The rookie has looked impressive in the majors so far, posting a 2.20 ERA and 12.1 K/9 over 16.1 innings this season.