Castillo has been diagnosed with a Grade 2 lat strain and could miss six weeks as a result, Dennis Lin of The Athletic reports.
This is a tough blow for Castillo, who suffered the injury during Tuesday's intrasquad game. While the Padres are hoping that he'll be able to pitch at some point during the abbreviated 2020 season, manager Jayce Tingler acknowledged that Castillo will "need some breaks" in order for that to happen.