Castillo (elbow) began a rehab assignment at Single-A Lake Elsinore on April 22 and has made three appearances for the affiliate, striking out five and working around a hit and a walk over three scoreless frames.
The 26-year-old southpaw has thus far looked good as he makes his return from his March 2021 Tommy John surgery. He re-signed with the Padres on a minor-league deal over the winter and will eventually report to Triple-A El Paso once he completes his rehab assignment.