Zambrano has been on the disabled list since Aug. 8 when he was scratched minutes prior to a start against Colorado due to back spasms. The move was retroactive to Aug. 2, when Zambrano left a game against Florida after three innings.
He threw a 45-pitch bullpen session before Sunday's rainout, will have another session Tuesday and then will make a rehab start Thursday with Class A Peoria.
"Remember, he hasn't pitched but three innings in over three weeks," manager Lou Piniella said. "He hasn't run the bases or swung the bat or done anything, so we've got to be a little cautious with him. So by sending him out, he can throw 70, 75 pitches and then he'll be ready for his start here [a week from] Tuesday."
To ensure he stays in the rotation, Zambrano will have to dedicate himself to working out his abdominal muscles.
"There's things in life you don't like that you have to do," Zambrano said. "I don't like to do abs, but I'll have to start doing my abs every day."
The team's medical staff has told Zambrano that strengthening his abdominal muscles and core will help him keep his back strong.
"Like the doctors say, I'm a strong man, a big man, and that's what big men do," Zambrano said. "Wherever they are weak, they get stronger."
After Zambrano came out of the game against Florida, he felt normal and believed he could make his next outing against Colorado. But while he was stretching before the game, he felt a twinge in his back that got progressively worse and eventually led him to pull himself out of the lineup.
Zambrano felt good during his bullpen session on Sunday, during which he threw all of his pitches.
"I was throwing it like I was pitching in a game to test my back, and nothing happened," Zambrano said. "Everything was fine."
Andrew Simon is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.