"We're going to stretch him out to get him back in the rotation," Piniella said Tuesday. "We'll see exactly when he's ready. I don't know how long it's going to be. Let's see how long he needs. We'd like to get him two or three appearances out of the bullpen stretched out."
Asked who would come out of the rotation, Piniella agreed that all of his starters have done well.
"We'll look at that situation and make a determination," Piniella said. "Let's hope it's a real, real tough choice."
All five starters have pitched decently, and there have been no decisions made on who will come out of the rotation when Zambrano is ready.
"We're not five games under .500 because we've had a bad rotation," general manager Jim Hendry said.
Zambrano's numbers in the bullpen were not good. In 8 2/3 innings, Zambrano gave up six earned runs and the National League hit .380 against him in the bullpen. The beginning of the end for Zambrano's setup days was the Pittsburgh Pirates' Garrett Jones' three-run homer off him on May 14.
"We took a chance that we needed to take," Piniella said. "We thought his velocity would increase. It hasn't."
Hendry said he was happy Zambrano gave the team his best effort in the 'pen.
"He gave it a good whirl, but the last couple of times out he said he wasn't able to throw as hard as he's used to or wanted to," Hendry said.
Piniella said before Tuesday's game that Zambrano would be available that night as a long man and could throw upwards of three innings if needed. Zambrano did get in the game, throwing a perfect ninth in Chicago's 6-2 win.
Zambrano had 91 victories in his first six full seasons as a Cubs starter before winning only nine times last year. He came to camp this spring in his best shape ever but got rocked by Atlanta on Opening Day and was 1-2 with a 7.45 ERA in four starts.
The combination of Zambrano's struggles, Ted Lilly's return from the disabled list and the ineffectiveness of every pitcher Piniella employed as setup man to closer Carlos Marmol led the Cubs to move their one-time ace to the bullpen.
But Zambrano didn't have very good command or velocity, rarely reaching 90 mph with his fastball. In his most recent outing, he gave up three runs in an inning against Pittsburgh. He had a 6.23 ERA in eight relief appearances, making a bad situation worse.
Zambrano, who turns 29 on June 1 and is signed through 2012, wasn't thrilled about leaving the rotation in the first place. So he was excited to hear he was returning.
"I'm happy, man," he said about becoming a starter again. "As long as the team's happy, I'm happy. Whatever this team wants me to do, I do."
Zambrano recently told Hendry he was having trouble getting loose quickly in the bullpen -- a must for a short reliever. And the Cubs aren't about to use a $91.5 million pitcher as a mopup man in one-sided games.
"No, no, no," Piniella said. "We don't need Zambrano or any pitcher of that quality in that role."
Hendry said he hopes the break from the rotation will energize Zambrano and help him return to "the dominant Z" of a few years ago. At his best, Zambrano could throw 95 mph.
Meanwhile, the Cubs believe Sean Marshall has emerged as a viable setup man for Marmol.
"He's flourished," Piniella said. "He's probably pitching as well as any left-hander in baseball out of the bullpen."
Bruce Levine covers baseball for ESPNChicago.com. Information from The Associated Press contributed to this report.