Manager Lou Piniella won't reveal who is changing roles, but the move will be made for the Cubs' next road trip in Pittsburgh, which starts on Monday.
"I don't think anybody deserves it," Piniella said, "but we're not going to have a six-man rotation. Zambrano probably didn't think he deserved it when we took him out. We're in a situation where we have to do something.
"I think truthfully that our starters have done a nice job here for the most part. At the same time, we're going to put Zambrano back in the rotation and we have to create a spot for him."
Zambrano moved to the bullpen on April 22, when Ted Lilly was activated from the disabled list. He has made 11 relief appearances, but was asked to return to the rotation because he didn't feel comfortable in the bullpen.
Carlos Silva is 6-0, and Lilly, Ryan Dempster and Randy Wells have been the most consistent starters. Gorzelanny took the loss on Wednesday against the Dodgers, giving up seven runs on seven hits over five-plus innings. He has started 81 of his 98 Major League games, and last year was the first time he pitched more in relief than as a starter. He was 3-1 with a 5.19 ERA in nine appearances with the Pirates before joining the Cubs on July 30.
"Just because somebody is going to be taken out of the rotation doesn't mean you can't help this team out of the bullpen," Piniella said. "Our bullpen could use all the help it could get."
The Cubs relievers rank 12th in the National League with a 4.48 ERA.
Sean Marshall is the perfect example of a pitcher who has accepted being a reliever -- and thrived. Marshall competed for a spot in the rotation this spring, but he's now 4-1 with a 1.93 ERA and the No. 1 setup pitcher for the Cubs.
"This is a guy who's shown to be very unselfish, and he's done a heck of a job," Piniella said of Marshall. "That's what you need when you start talking about a team. You need people who are unselfish and will do the job that's necessary.
"We can't pitch six people -- we cannot pitch six people. We'll pitch five and do what we feel is best and go from there. Just because we do something doesn't mean we can't readjust later on if there's an injury or whatever."
If Gorzelanny is the one who goes into the 'pen, the Cubs will have an overload of left-handers. He'll join Marshall, John Grabow and rookie James Russell. Piniella said the decision won't be based on whether the team needs another lefty or right-hander.
But Zambrano has to hold up his end of the deal, too. He was 1-2 with a 7.45 ERA in four starts before the switch.
"What I'm hoping for is when Zambrano goes into the rotation, he pitches the way he's capable of and runs off some ballgames for us and wins," Piniella said. "That's really the most important thing, and he's certainly very capable of it. The way he's throwing now [out of the 'pen], just carry that forward and he'll get some good results."