Zambrano, on the 15-day disabled list with back spasms, threw a side session Sunday and is expected to throw another side session Tuesday before his rehab start for Peoria.
He was scheduled to pitch against Colorado on Aug. 7, but was scratched 15 minutes before the game with stiffness in his lower back. Back problems also limited Zambrano to just three innings in his start against the Florida Marlins on Aug. 1.
Zambrano, who is 7-4 with a 3.35 ERA, said he will work on strengthening his abdominal muscles to help prevent further back problems.
"I got to be honest with me, [media] and the fans. I'm 28 years old. I'm not 16 anymore," Zambrano said. "I'm a big guy. I work out every day hard. One of the things I got to do is work on my abs. I have to do more abs to make my back more stronger."
The Cubs will keep Zambrano around 75 pitches in his rehab start.
"Remember he hasn't pitched more than three innings in over three weeks," manager Lou Piniella said. "He hasn't run the bases or swung the bat so we have to be a little cautious with him."
Piniella will welcome back left-hander Ted Lilly off the disabled list on Monday night at San Diego. Lilly is scheduled to start and Piniella said he will be limited to around 85-90 pitches.
Lilly, who is 9-7 with a 3.59 ERA, was on the disabled list with left shoulder inflammation.