DENVER -- The Chicago Cubs moved quickly following Carlos Zambrano's scratched start Friday night.

Without waiting for the results of an MRI scheduled in Chicago over the weekend, the Cubs placed Zambrano on the 15-day disabled list Saturday, retroactive to Thursday, and recalled right-handed reliever Jeff Stevens from Triple-A Iowa.

Zambrano appeared fine until beginning his warmup 15 minutes before Friday's scheduled start to open a four-game series here. He left the field in discomfort, saying his back tightened up and he was getting spasms.

"He went home today," general manager Jim Hendry said before Saturday's game. "We put him on the DL. He's getting an MRI either tonight or tomorrow morning, but you've had two [incidents] like that in a row, so obviously something's wrong. And at the same time, you get to be pitching short."

Zambrano lasted only three innings in his previous start, last Saturday in Miami, and left that game with lower back stiffness. He had been symptom-free up until his pregame warmup Friday.

"He's had some on-and-off issues in the past, but he was fine all week," Hendry said. "I talked to [Cubs director of athletic training] Mark O'Neal every day. I saw [Zambrano] in Cincinnati. He was fine. He took BP."

Given the fact that Zambrano felt fine up until batting practice then had the back problems when he started his warmup throws shortly afterward, there is some concern that his typically vigorous displays in the batting cage could be related to the back problem. Manager Lou Piniella indicated that if the MRI suggested a connection to Zambrano's batting practice, that part of his routine would be "curtailed."

The Cubs got immediate help by recalling Stevens from Triple-A Iowa to join the bullpen, and long man Jeff Samardzija will take Zambrano's spot the next time through the rotation. Samardzija pitched four innings of relief in Friday night's loss to the Rockies, throwing 73 pitches.

"Samardzija will pitch at home against the Phillies on Wednesday," Piniella said. "He threw the ball well last night. [Ted] Lilly will go on a rehab [assignment] in the middle of the week. I think by the time we go to the West Coast in San Diego, he'll be able to join the rotation."

Lilly is rehabbing from right-shoulder inflammation and minor arthroscopic surgery on his left knee, and is eager to get back and help the team. Asked if Zambrano's trip to the DL gave him a heightened sense of urgency to get back sooner, Lilly balanced his desire to help the Cubs in their pennant push with the knowledge that he has to be ready to pitch well and stay healthy when he returns.

"I'm going to do what I can do," Lilly said. "I need to prepare to handle the load that's coming up here in the most important part of the year. I want to be as strong as I can possibly be. It's certainly a lot more important to me to finish strong than it would be to get off to a good start."

Lilly has no problem with the decision to send him down for a Minor League rehab assignment, and Hendry was adamant that Zambrano's absence would not figure into Lilly's timetable.

"We're not going to rush Teddy," Hendry said. "Because, really, I'm not worried about the knee scope. You worry about his shoulder, and you don't want to get [the issue] going again. He's had no problems as long as we've had him."

Zambrano will be eligible to be reactivated on Friday, Aug. 21 in Los Angeles. This is his second DL stint of the season, having suffered a left hamstring strain that sidelined him May 4-22.

The Cubs have been plagued by health concerns throughout the season, and the consistent trips to the DL have forced Piniella to continually juggle his plans for the pennant race.

"We've had physical problems here all year long and for sustained periods of time," he said. "It doesn't make it any easier, but at the same time, you can't complain about it. They're part of the game. You got to go out and compete regardless.

"I said at the All-Star break that the secret for our ballclub is to get healthy, stay healthy and score runs. I've said that, and I cling to it."