Zambrano returned to the Cubs on Friday after missing more than a month after a suspension and time on the restricted list following his dugout tirade against teammates during a loss to the Chicago White Sox on June 25.
After a stint with Triple-A, Zambrano returned to the Cubs on Friday and has pitched out of the bullpen twice and will be available Wednesday against the Milwaukee Brewers.
The Cubs' Opening Day starter, Zambrano won 91 games between 2003 and 2008 but slumped to 9-7 last season and is 3-6 with a 5.71 ERA and was demoted to the bullpen earlier this season.
"I've been in this game for nine years," Zambrano said. "For seven years, I've been doing a good job and the last few years unfortuiantely I haven't. But it's not [too] late. It's not [too] late. We still have two months to go to try to finish over .500."
Piniella said the Cubs will hold Zambrano to 75-80 pitches on Monday. Zambrano threw 53 pitches in his last outing on Sunday against the Rockies.
"Sometimes you don't know what you have until you lose it," Zambrano said of starting again. "I don't want to lose this opportunity. I just want to take advantage [of it] be prepared and be ready for the last two months."
Zambrano made it known immediately upon his return to the Cubs that he would like to start again if the Cubs were willing. After the trade of Ted Lilly to the Los Angeles Dodgers and Carlos Silva going on the disabled list after a heart scare in his last start, the Cubs had openings.
Rookie Thomas Diamond filled the other spot in the rotation for now after striking out 10 in his debut on Tuesday against the Brewers.
ESPNChicago.com's Bruce Levine contributed to this report.