Zambrano's next turn in the rotation would have been Friday, and Cubs manager Lou Piniella said the team would most likely call up right-hander Randy Wells for that game, which will open a three-game series in Milwaukee.
Zambrano injured his left leg running to first base Sunday in the fifth inning against the Florida Marlins trying to beat out a bunt hit.
"It was a normal run and three steps before I got to the bag, I felt a pop in my hamstring," Zambrano said Monday. "I tried to hustle with a flat tire at that moment, I tried to get to the base. When I crossed the base and was safe, I felt more and more pain."
He convinced Cubs athletic trainer Mark O'Neal and Piniella that he could stay in the game, but was ultimately lifted. Zambrano also tried to talk the Cubs brass out of placing him on the disabled list.
"Obviously, he doesn't want to go on the DL, which is a part of him that we like," Cubs general manager Jim Hendry said. "But at the same time, the doctors feel he needs two, 2 1/2 weeks to be 100 percent. There's no reason to push him and it would be foolish on our part to push it because it could get worse. The easiest situation is to take care of it now."
Zambrano, who underwent an MRI Sunday night, understood, even though he felt better Monday.
"I want to be out there to help my team," Zambrano said. "Any injuries that happen to any ballplayer, they feel sad and disappointed. But it's good that it wasn't Grade 3 damage, it was Grade 2. I can take it easy and the good thing is to get healed. The hamstring needs rest."
Zambrano even tried persuading Piniella that he was OK.
"Z wanted to throw a bullpen," Piniella said. "He wanted to throw a bullpen tomorrow. We did the right thing. The danger with this thing is [if he comes back] too early, you hurt it more and now it's a six- to eight-week thing and that we can't have."
The Cubs were one of two teams that began the season without a player on the disabled list. Zambrano was on the DL last year because of a right shoulder strain.
"I don't like being on the DL, believe me," Zambrano said. "I hate being on the DL. I'm a guy who can play with pain. I've done this before. Sometimes my arm hurts and I'm in pain and sometimes my leg doesn't feel good and sometimes I have a headache -- that's pain also -- and I still pitch and still run and do my job. People have to play with pain. The body isn't all the time 100 percent."
If all goes well, and Zambrano only needs the two weeks, he could be back in the rotation for the May 19-21 series at St. Louis.
"When he's eligible to come off the DL, he'll be able to pitch that week, whether it's the beginning of the week or toward the end of that week," Piniella said.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.