Castro will undergo an MRI on his right shoulder, Bryan Hoch of MLB.com reports.
Castro hasn't pitched since Sunday, when he allowed four runs (two earned) on two hits and a walk while failing to record an out against the Red Sox. His injury likely contributed to his struggles, and an MRI should help provide a better idea of the severity of the issue. It's not yet clear whether the right-hander will be available again prior to the All-Star break or whether he'll require a trip to the injured list.