The Cubs right-hander underwent an MRI on Tuesday after complaining of stiffness in his back the day before. Zambrano had to leave Monday's game after 1 1/3 innings because of back spasms.
Zambrano has a history of back problems, and Cubs athletic trainer Mark O'Neal said they were able to compare past MRIs with the latest one.
"There's no significant irritation to his lower back, there's nothing that's changed significantly," O'Neal said Tuesday. "There's no spinal cord damage, there's no nerve root irritation. There's some muscular skeletal spasms that are protecting what you would call early signs of degeneration in his low back, which athletes are prone to get."
The Cubs have 3 1/2 weeks left of the season, and Zambrano's status will be evaluated daily. He won't be allowed to consider another start until he has a pain-free bullpen session, O'Neal said.
"He still has some irritation in his low back and we'll treat that," O'Neal said. "If it takes two days, three days, a week, we can't answer that. We're not going to put him in a position to pitch in a game until he's able to do that."
Zambrano, who is 14-6 with a 3.50 ERA in 30 starts, was relieved that his problems weren't serious.
"I want to pitch," Zambrano said. "I can't risk my career for this. When you have back problems, like when I was throwing the other day, you can hurt your shoulder and that could be the end of my career. I don't want that to happen. It's nothing big right now."
The injury will definitely hurt Zambrano's chances at winning the National League Cy Young. He ranks among the NL leaders in ERA, wins, strikeouts (188), road ERA (2.79), and opponents batting average against (.212).
"Every pitcher has a goal and [winning the Cy Young] was one of my goals," Zambrano said. "Now I'm close to getting that. Like I said, I can't pressure myself or risk myself. You have to take care of yourself and be smart."
Zambrano said one of the factors contributing to his back problems is that his right leg is longer than his left.
"We're born like that, all Zambranos are born like that," he said. "My dad has a problem with that, too."
|"I can't risk my career for this. When you have back problems, like when I was throwing the other day, you can hurt your shoulder and that could be the end of my career. I don't want that to happen. It's nothing big right now."|
|-- Carlos Zambrano|
"It's always there," he said. "It's like the doctor told me in the office this morning, 'You'll always feel it if you're running or riding the bike or playing with your daughter or playing basketball -- after that you'll be sore."
O'Neal said Zambrano is not unlike other players who need to take care of their core.
"It's a career maintenance thing for every player," O'Neal said. "Every player needs to be doing core strengthening work and work on things to stabilize their lower back. There's not a player out there who doesn't need to address low back stabilization."
Zambrano admitted after Monday's game that he probably shouldn't have been pitching because his back was tight. Cubs manager Dusty Baker wasn't surprised Zambrano tried to gut it out.
"That's natural, actually, especially for a guy who's a gamer," Baker said. "In the old school, that's how it was. It's a matter of how hurt a guy is. Nobody is out there without hurting this time of year, somebody has something wrong. It's just a matter of degrees."
Zambrano's next scheduled start is Saturday against Atlanta. If he can't go, the Cubs will likely tab another rookie, possibly right-hander Jae Kuk Ryu. They already have four rookies in the rotation.
Would Baker advise Zambrano to take the rest of the season off?
"I'm not the one making the decision," Baker said. "I'm not the doctor."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.