"It's just a tight back and stiff and I just have to treat it before my next start," Zambrano said.
He gave up two runs on three hits and walked three in his shortest outing of the season.
What caused the back problems?
"Sometimes I sleep on the wrong side of the bed, or maybe the bed is too soft at the hotel," Zambrano said. "Anything can do it. Sometimes [Derrek Lee] feels things in his neck. We're big men. We suffer from neck and back injuries. We just have to take care of things in the trainer's room and get ready for the next start.
"It's nothing to worry about," Zambrano said. "Let's treat it and take care of it."
In the third inning, Zambrano hit Florida's Hanley Ramirez on the left knee with a pitch, then got the next two batters out. Cubs manager Lou Piniella went to the mound to talk to his starter, who stayed in the game and gave up a RBI single to Jeremy Hermida, then struck out Cody Ross.
"As the game was going, it was feeling worse and worse, and I saw my arm dropping," Zambrano said. "Lou told me something that was right. He said, 'Before you get injured because of your back, let's get you out of the game.' I knew my release point was low. It was the best choice to take me out of the game."
Zambrano (7-4, 3.35 ERA) entered the game having won his past three decisions. He did say he had a little discomfort in his start against St. Louis on July 12. In that game, he didn't alter his release point or arm angle. On Saturday, he was almost throwing sidearm.
The Cubs opened a 6-0 lead before Zambrano left, but then had to rally to win, 9-8, in 10 innings.
"Today we won the game, and that's the most important thing," Zambrano said. "It's good to have good pitching and good relievers."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.