Castillo (elbow) has allowed one hit and no walks while striking out two over 2.2 scoreless innings across three appearances with Triple-A El Paso since being activated from the 7-day injured list May 10.
Castillo opened the season on El Paso's injured list while working his way back from March 2021 Tommy John surgery, but he proved his health during a recent five-game rehab assignment at Single-A Lake Elsinore. The 26-year-old lefty could eventually earn a call-up to the San Diego bullpen now that he's healthy again.