Instead, it will be Angel Guzman, a Sept. 1 call-up who has appeared in only five games -- all Cubs losses -- and has a 7.04 ERA in 7 2/3 innings.
The Brewers are tied with the Mets for the NL wild card. Zambrano was expected to pitch a couple of innings as a playoff tune-up, but the Cubs changed their minds and now Milwaukee won't have to face one of the top pitchers in the National League.
Instead, the Brewers get a right-hander who had surgery on his pitching elbow in the spring and missed most of the season.
Cubs manager Lou Piniella said Zambrano told team officials he'd rather throw a side session in the bullpen instead of working the first couple of innings Sunday. If Zambrano had started, he would have been pitching on three days' rest.
"He would rather long toss," Piniella said. "That's his preference. He feels that's a better scenario for him, and what we're interested in is what's best for him."
Zambrano is scheduled to start Game 2 of the division series Thursday at Wrigley Field.
Guzman has given up nine hits and four walks while striking out six this season. He pitched in seven minor league games for three teams on a rehab assignment, posting a 1-0 record with a 1.59 ERA.
Piniella said it was likely that no Cubs pitcher would throw more than two innings Sunday.
"We'll have to pitch as many people as we can," he said. "We're in a situation where we've used six starters this week and there's really not much of a choice."
Piniella rested most of his starting lineup in Saturday's 7-3 victory, but said he would start his regulars Sunday against lefty CC Sabathia.
"It will be interesting to see what happens," Piniella said. "It's amazing to play 161 games and it comes down to the last game. It's unbelievable."