Zambrano moved to the bullpen in late April, and he'll need at least two longer relief appearances to get him "stretched out" to start, according to manager Lou Piniella.
"The reason we put Carlos in the bullpen was because we talked about it as a staff, and we felt that with his experience, his intimidation factor and the fact that he's pitching one inning, that he was most suited out of our rotation to do that," Piniella said.
It didn't work out.
Zambrano never regained the velocity on his fastball that he once had as a dominating starter so the Cubs decided to end the experiment.
"It was a chance we needed to take," Piniella said. "We thought his velocity would increase, and it hasn't. I said this wasn't going to be permanent when we did it. I appreciate the fact that he went out there and gave it his best shot."
Zambrano went along with the program, albeit grudgingly. However, he said Monday night he'd go along with anything the Cubs wanted to do.
He was 1-2 with a 7.45 ERA as a starter and moved to the bullpen when lefty Ted Lilly came off the disabled list in late April. Zambrano worked a 1-2-3 ninth inning in Tuesday's 6-2 win over the Rockies in a non-save situation.
The Cubs say they have not decided who will go from the rotation to the bullpen when Zambrano comes back.
CUBS 6, ROCKIES 2: Carlos Silva worked six innings to improve to 5-0 as the Cubs won their third straight. The offense scored three in the fourth inning and three in the eighth, bunching three hits in each of those innings. Ryan Theriot put the game out of reach with a two-run single in the eighth.
--Sean Marshall is the team's latest eighth-inning setup man. Marshall assumes the role with Zambrano transitioning from reliever back to starter. Marshall, who competed for a starting job in spring training, has been the best setup man, outperforming veteran John Grabow.
"I don't know too many people who have pitched in the eighth inning in the game better than him," GM Jim Hendry said of Marshall.
--Tyler Colvin got his second straight start after seeing bench duty for a week. He made the most of it with three singles and an RBI. Colvin also stole the first base of his career. The Cubs are 8-4 when Colvin starts. His play has been limited because veteran outfielders Alfonso Soriano, Marlon Byrd and Kosuke Fukudome have played well.
--Starlin Castro has reached base safely in 10 of his 11 games. He picked up his first Wrigley Field RBI with a forceout in the fourth inning. He also singled home a run in the eighth.
--Silva became the first Cubs pitcher since Greg Maddux in 2006 to start a season 5-0. Silva worked six innings and turned in his sixth quality start.
"I'm never afraid to throw strikes," he said. "I don't like to walk anybody. Of course you want to make your pitches, too."
It's the second time in his career he's begun 5-0. He did it with the Twins in 2004.
--Geovany Soto walked three times, raising his on-base percentage to .463. It was the sixth multi-walk game of the season for Soto, who has been selective at the plate in both the seventh and eighth spots. He leads the team with 29 walks and is on pace for 113. He had 50 walks last season.