Zambrano, a 13-game winner this season, is 1-1 over his last five starts and twice failed to get out of the fifth inning. He has seen a drop-off at times in his velocity.
Leading the NL Central and with the best record in the majors, the Cubs want a fresh Zambrano for the final month. He went through a similar tired arm period a year ago when he was winless in August before rebounding to go 4-1 in his final five regular-season starts.
Pitching coach Larry Rothschild said Zambrano isn't in any pain and his arm feels good.
"It's just that he's gone through this every year the last four or five years, at a point in time where his arm feels a little bit heavy, tired," Rothschild said. "I think everybody goes through that, I think it's a little bit of a dead arm period.
"He has felt a lot better the last couple of days ... As he sees everything coming back, he'll get better. He'll get on his stride."
Sean Marshall started the finale of a four-game series Sunday against the Phillies at Wrigley Field.
Zambrano's next start will come Tuesday against the Astros at Wrigley Field with Ryan Dempster pitching Wednesday, also giving him extra day's rest.
By giving Marshall the start and moving Zambrano back, the Cubs also gave right-hander Rich Harden -- who pitched only five innings in his previous start Friday against the Phillies -- extra rest. Harden had been slated to start Wednesday but now might not pitch against until a series against St. Louis that begins on Sept. 9, manager Lou Piniella said Sunday.
The Cubs' staff will be bolstered by callups when major league rosters expand on Monday. They also will activate veteran Jon Lieber off the disabled list.
Zambrano, who got a $91.5 million, five-year contract last August, played long toss Sunday but didn't comment after the workout.
On Saturday he said he was in no pain, unlike when he went on the disabled list in June with a sore shoulder.
"I just have to wait until my arm strength and power come back," he said Saturday. "Not only me. It happens to other pitchers, especially when you are a power pitcher. The most important thing and the thing to learn here is to know how to pitch with this. ... I think this is the time when you have 170 innings, close to 180 innings, you have to do whatever it takes to rest your arm."
Rothschild said Sunday's switch should set up the Cubs' rotation for the final month's playoff push.
"We're looking at this over the long haul," he said. "It's the right thing to do, no matter what."