Zambrano was taken to Northwestern Memorial Hospital to be evaluated, and Cubs manager Mike Quade said he didn't know if Zambrano will have to miss time.
"This has been an ongoing thing from time to time with him," Quade said.
Zambrano wasn't feeling right when he arrived at the ballpark.
"He got here this morning, [and] I think he was a little cranky when he talked to us," Quade said. "They worked on him extensively hoping he could loosen it up and give us what he could and hoping they wouldn't bunt on him because that was going to give him trouble bending over too much. And he threw quite a few balls up. He just didn't have the freedom and flexibility that he normally does."
It was a bunt-like squib by Brandon Crawford that might have worsened Zambrano's condition. Crawford, the second batter in the second inning, bunted, and Zambrano reached for the ball but couldn't get to it, so he raced to first to cover. Carlos Pena flipped the ball to the pitcher but it went behind Zambrano's head. Zambrano then retrieved the ball and threw to second to try to get Nate Schierholtz, but his throw was wild for an error.
After Zambrano gave up a run-scoring single to Miguel Tejada, his first pitch to Eli Whiteside was high and 87 mph. Catcher Koyie Hill then went to the mound as did athletic trainer Mark O'Neal, and Zambrano was pulled.
For the game, Zambrano threw 34 pitches and gave up one unearned run, two hits and walked two while striking out one. It was his shortest start since he lasted one inning June 25, 2010, against the White Sox.
"I don't know what [caused the injury]," Quade said. "He was bothered going in but felt like he did enough in his pregame, in his side [session] to get ready for the game, that he could take a shot. And I'm convinced he was taking a shot at way less than 100 percent. And that activity that inning, he bobbled the bunt and made a quick throw, that didn't help, I'm sure."
Quade said it was Zambrano's decision to go ahead and start.
"It was up to him," Quade said. "You don't want anybody to hurt themselves or anything, but it was pretty incredible the work [the training staff] did, and he felt like he was OK. And no one knows himself better than him."
Hill talked to Zambrano before the game.
"I told him if he needs a break just give me a heads up," Hill said. "I think that bunt play was a tough play to recover from."
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. Alex Ruppenthal is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.