"They took him to the hospital to get an X-ray or an MRI or whatever, just to clear out everything," Renteria said. "We don't have the results yet. We'll know tomorrow."
Castillo was supplanted by callup Rafael Lopez, who went 1-for-3 with his first career hit, a walk and RBI -- with the hit coming against Dodgers ace Clayton Kershaw.
It's likely Lopez will be the starting catcher if Castillo is sidelined.
"If they put me back there, I'm obviously going to be ready, mentally and physically," said Lopez, who hit .285 with 27 RBIs at Triple-A this year. "Obviously, I want to be back there. I also understand how the game works and how September callups work and all that. But I definitely would love to be back there. You never know what happens."
Cubs trainers met Castillo and starter Edwin Jackson on the mound just after Hanley Ramirez doubled to right in the first. The meeting was initially thought to be regarding Jackson, who was making his first start since Aug. 20 after returning from the disabled list. But replays showed Castillo getting grazed by a pitch during Ramirez's at-bat, which appeared to cause the injury.
Daniel Kramer is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.