Lilly, the Cubs' only All-Star representative, was scratched from his scheduled start on Saturday against the Cincinnati Reds. The Cubs expected to have test results on Saturday.
"From talking to Lilly, he had this when he was in Toronto and it took him a couple weeks," said Cubs manager Lou Piniella, who didn't know about the pitcher's injury until late in batting practice Friday. "There's a possiblity there [that Lilly has to go on the DL]. I wouldn't say it's for sure, but it's a possibility."
Lilly was coming off a loss to the Philadelphia Phillies on Monday in which he gave up a career-high nine runs on eight hits over four innings. Lilly (9-7, 3.59 ERA) also served up two homers in the game. That start was his first in nine days, and had been pushed back because the left-hander had some inflammation in his left knee.
Cubs pitching coach Larry Rothschild told Piniella during batting practice on Friday that they needed to talk about the rotation. Piniella thought the topic was Ryan Dempster, who will be activated from the disabled list for Tuesday's game and is in line to make that start. Dempster has been on the disabled list since July 8 because of a broken right big toe.
"It seems like every time we get a player, we lose one," Piniella said. "Let's see what the MRI says on Theodore and then we can go from there."
Lilly's injury is the front part of his shoulder, Piniella said, and could be tendinitis. The lefty was on the DL twice in 2005 because of tendinitis.
"Obviously, Teddy is a huge part of any success we've had the last few years, or certainly a key component to the future," Cubs general manager Jim Hendry said. "Hopefully, it won't be anything serious. We probably won't know the results of the test until [Saturday]."
Kevin Hart (1-1, 2.30 ERA) will make his third start on Saturday, and Rich Harden (6-6, 4.76 ERA) will start Sunday against the Reds. The Cubs had hoped to keep Harden on a night game schedule because he pitches better under the lights.
Carlos Zambrano will be moved up to Monday, and will be followed by Dempster. X-rays of Dempster's toe Thursday showed it had made progress and Dempster (5-5, 4.09 ERA) lobbied Piniella to start this weekend.
"He wants to pitch Sunday," Piniella said. "I don't think we're going to be able to do that. I think we'll be prudent and wait until Tuesday."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.