Ted Lilly breathed a sigh of relief Saturday after an MRI arthrogram revealed only inflammation in his left shoulder, but he still is expected to spend at least three weeks on the disabled list.

"I didn't think there would be any significant tears," Lilly said. "I've been in this situation [before] and have a good idea of what's going on.

"It's very similar to what was going on in Toronto in 2005, and I was able to give it a rest, let the inflammation subside, do my rehab and take care of it. Then I came back and was throwing the ball without any pain."

Lilly also will undergo arthroscopic surgery Monday on his left knee, killing two birds with one stone while on the DL. He said he doesn't think pitching with the knee problem contributed to his shoulder injury.

"With the exception of the last start in Philly, I was pitching pretty good," he said. "I didn't feel as though I was compromising my delivery because of my knee."

With Ryan Dempster returning from his broken toe Tuesday night, Kevin Hart will remain in Lilly's rotation spot.

"We'll have two rookies in the rotation," manager Lou Piniella said. "It's, uh, well ... we'll see what happens."

Hart pitched well again Saturday, and fellow rookie Randy Wells has been solid since coming up.

"Look, there's never a positive when a player goes on the DL," Piniella said. "We need to just keep hanging in there, and we're going to have some fresh arms at the end of the year."

Piniella briefly considered moving Jeff Samardzija into the rotation, and thought seriously about switching left-handed reliever Sean Marshall. Though the Cubs can't afford to lose their only left-handed reliever on the roster, they're keeping their options open.

B.J. Ryan's debut with the Cubs may come earlier than expected. He will pitch at Triple-A Iowa on Monday and could report to the Cubs next weekend. If the left-hander can move into Marshall's role, the Cubs could slide Marshall into the rotation while Lilly heals.

Do the Cubs need a lefty in the rotation?

"It'd be nice to have one, but you do the best you can quality-wise, and that's what we need to do," pitching coach Larry Rothschild said.