Herrera said he felt fine warming up in the bullpen, but he said he started experiencing it when he took over for Jason Vargas.
"I feel pretty good. It's just a tight forearm; it makes my fingers feel like numb," Herrera said. "I'm going to have an MRI tomorrow. They probably don't want me to pitch tomorrow, but I think I'm going to be OK for Sunday."
With the score tied at 2, Herrera came into the game to start the seventh and walked third baseman David Freese on five pitches. After Freese headed to first, Yost and trainer Nick Kenney went to the mound to confer with Herrera.
After a short conference, Herrera came out of the game and was replaced by rookie left-hander Brandon Finnegan.
"I just didn't feel those two fingers [index and middle], and I didn't feel where the ball was going," Herrera said. "I don't think it's anything bad, just a tight forearm."
T.R. Sullivan is a reporter for MLB.com. Read his blog, Postcards from Elysian Fields, and follow him on Twitter @Sullivan_Ranger This story was not subject to the approval of Major League Baseball or its clubs.