Zambrano also felt some cramping in his hamstring when he walked to the mound to warm up in the Pirates' seventh. He served up a leadoff homer to Pittsburgh's Garrett Jones and had a 2-2 count to Ronny Cedeno before athletic trainer Mark O'Neal went to the mound to check on the right-hander.
"I was kind of worried about it," Zambrano said. "I threw that pitch [to Jones] with nothing [on it]. Hopefully, we can work on this and take care of this before my next outing."
"It's nothing serious," Cubs manager Mike Quade said. "I guess he thought he could get through it. I wish we'd found out about it and made a decision about it before the home run."
Zambrano was removed after throwing 99 pitches, 61 for strikes. He gave up three runs on five hits and three walks over six-plus innings. The Cubs trailed, 3-0, when he was lifted before rallying.
"It's no big deal," Zambrano said. "I thought I could get away with that. Obviously, that wasn't the case."
Quade wasn't upset that Zambrano didn't tell the team he wasn't feeling 100 percent.
"You want guys to say, 'I got it, I'm going to do this,'" Quade said. "It's a fine line. If you can't, you can't. They've been honest with me. I love the fact he wants to go back out there, but we're talking about the first start of the year, and there's a lot of baseball left to be played."
Zambrano has been bothered by cramps in the past in his forearm and hand.
"You just deal with it," Quade said. "If he'll throw 100 pitches and give up two or less runs every outing, he can cramp up all he wants."
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter@CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.