Castillo was struck by a foul ball in the first inning on Friday. A trainer and manager Rick Renteria went to check on him, but Castillo managed to finish the top of the first.
"He's being evaluated," Renteria said after the Cubs' 14-5 loss. "They took him to the hospital to get an X-ray or MRI to clear out anything. We'll know more tomorrow (Saturday)."
Rafael Lopez hit for Castillo in the bottom half, and then stayed in the game to catch.
Castillo is hitting .238 with 11 home runs and 42 RBIs this season.