ST. PETERSBURG -- Cubs pitcher Carlos Zambrano was to undergo an MR arthrogram on his right shoulder on Friday, and was expected to miss his next scheduled start Tuesday against Baltimore.

"We're not looking at Zambrano at all for Tuesday," Cubs manager Lou Piniella said.

Zambrano had to come out of Wednesday's game against Tampa Bay because of discomfort in his right shoulder, and flew back to Chicago on Thursday to be examined. Team orthopedic specialist Dr. Stephen Gryzlo was to oversee the MR arthrogram test and do other examinations, as well, on Friday. An MR arthrogram is more detailed than an MRI (magnetic resonance imaging) test.

"I hope it's not serious, and I hope we can find out what's wrong and we can take care of it in short order," Piniella said.

This shoulder injury is new, team officials said, and not related to the neck stiffness Zambrano experienced on May 17. In that game against Pittsburgh, Zambrano lasted four innings. In his next outing against the Pirates, he went seven innings and struck out six.

Among the candidates to sub for Zambrano in the Interleague game Tuesday against the Orioles is left-hander Sean Marshall, who started on Thursday for Triple-A Iowa. He gave up one run on four hits over eight innings, striking out four in a 4-1 win. The only run came on a solo homer in the eighth.

"He's a good possibility," Piniella said.

Zambrano is 8-3 with a 3.13 ERA in 16 starts so far, and ranks among the National League leaders in ERA, wins, games started and innings pitched.