CHICAGO -- Cubs pitcher Carlos Zambrano was examined Wednesday by the team's orthopedic specialist, but did not undergo an MRI exam, and the team is still trying to determine the extent of the problem.

Zambrano will miss his next scheduled start Sunday in Cincinnati, Cubs manager Lou Piniella said Wednesday. The right-hander, who has complained of a tired arm in August, left Tuesday's game against Houston after throwing 86 pitches over five innings.

Team orthopedic specialist Stephen Gryzlo examined Zambrano on Wednesday.

"The MRI that he was supposed to take he didn't take, for whatever reason, so the examination is incomplete," Piniella said. "Until he gets an MRI done, we don't have anything to report."

Zambrano may get the MRI on Thursday, which is an off-day for the Cubs, Piniella said. The right-hander, who was on the disabled list in June because of discomfort in his right shoulder, was 13-5 with a 3.58 ERA in 27 starts this year.

"Obviously, you're concerned," Piniella said. "He's a top-notch pitcher. Believe me, we sent him out there [Tuesday] thinking that he was perfectly fine to go out there. If not, we wouldn't have sent him out there."

Asked to assess Zambrano's performance on Tuesday, Piniella said his location wasn't too sharp. Zambrano gave up three runs on five hits and three walks, striking out three.

"He was out there competing, and it sort of surprised me in that bottom half of the fifth inning when he told the pitching coach he wasn't feeling well and couldn't go further," Piniella said.

Piniella had talked to athletic trainer Mark O'Neal, but had yet to talk to Gryzlo.

When pressed as to why Zambrano didn't have the MRI Wednesday, Piniella said, "I don't think Carlos was available today to take it."

Zambrano had not complained of any discomfort in his arm, but had some fatigue, and the velocity on his fastball had dropped.

"I don't have any answers for you," Piniella said. "I wish I did. I'd like to have clarification myself. I don't want to lie to you; I want to be as truthful as I can."