Zambrano was pulled after giving up three runs on five hits and three walks over five innings against the Houston Astros on Tuesday. He did not get a decision in the Astros' 9-7, 11-inning win.
The Cubs had given Zambrano two extra days of rest since his last start because he was feeling some fatigue in his arm in August, which resulted in a 1-1 record and 7.43 ERA in five games.
"He told our pitching coach he wasn't feeling good," Cubs manager Lou Piniella said after Tuesday's game. "He went up with the trainer, and he'll see Dr. [Stephen] Gryzlo tomorrow, and we'll know more by then."
Gryzlo is the Cubs' orthopedic specialist. What was Zambrano's problem?
"It has to be in his arm -- I don't know," Piniella said. "We'll know more tomorrow."
Zambrano, who is 13-5 with a 3.58 ERA this season, spent time on the disabled list in June because of soreness in his shoulder.
"If he can pitch, he'll pitch, and if he can't, we'll put [Sean] Marshall in the rotation and go from there," Piniella said. "That's the end of it."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.