Castillo (hamstring) will begin his rehab assignment Saturday with Triple-A El Paso, Kevin Acee of The San Diego Union-Tribune reports.
The right-hander was sent to the 10-day disabled list July 7 with a right hamstring strain and will initiate his rehab assignment in the Pacific Coast League. Castillo sports a 2.84 ERA, 0.79 WHIP, 18 strikeouts and four walks in 12.2 innings during his ascension into a reliable relief role with the big club, and if all goes well, he should start building on those numbers as a Padre sometime in the coming week.