Lilly was to have pitched last Friday for Triple-A Iowa, but he was scratched because of stiffness in his back.
"I pitched in the Minor League game [April 4], felt good, threw the ball good," Lilly said on Monday. "I came out of it and was a little bit stiff, and didn't think too much of it and woke up the next day and was stiff. I tried to throw a bullpen [session] on Tuesday, and I couldn't rotate. The more I moved, the worse it got.
"It's mid-back muscle tightness, and it got to the point where I didn't have any rotation. Obviously, I couldn't throw a baseball. Unfortunately, it's one of those things that takes a while."
Lilly, who was 12-9 with a 3.10 ERA last season, is rehabbing from arthroscopic surgery on his left shoulder, which he had in November. The shoulder hasn't been a problem, as Lilly missed time this spring because of the flu and recently has been slowed because of his back.
"I've been fortunate that way -- my shoulder and elbow and everything has been good," Lilly said. "I've felt great, to be honest. It's been some of the other things that have curtailed my progress."
As for his rehab start, Lilly said he'd prefer to face more experienced hitters and pitch for Triple-A Iowa. Manager Lou Piniella said after Monday's game that Lilly would start for the Iowa team and was expected to throw about 60 pitches.
"The way I've been throwing the ball, I feel like I've gotten a little bit better each time," he said. "My expectations are that I'm preparing myself to be able to pitch in this homestand, and I thought I was pretty much on track to do that.
"I expected to be ready to do well. Unfortunately, this [back] is another thing that's set me back a little bit. Fortunately, it's not September with the playoffs coming. On the other side, these games count, too. I want to get out there as soon as possible."
Starting pitching has been the one element that has not been a problem in the first week of games. Take the Opening Day 16-5 loss out of the equation, and Cubs starters have posted five quality starts, giving up five earned runs on 31 1/3 innings.
It's not certain which starter would lose his spot when Lilly does return.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.