Johan Santana warms up his arm during today's spring training...

Johan Santana warms up his arm during today's spring training workout at Digital Domain field. (Feb. 25, 2011) Credit: Newsday/Alejandra Villa

Johan Santana's schedule of rehab starts has been stopped indefinitely and a return this season is further in doubt after the Mets announced Thursday he is suffering from "fatigue" of the left shoulder.

Santana was examined at the Hospital for Special Surgery and the team's medical staff determined that his surgically repaired shoulder is "otherwise normal." That was considered encouraging news, but Santana, who will stay with the Mets for the next week, will be shut down from throwing until his shoulder is at "full strength," according to the team.

Santana did not have an MRI as part of the exam, which suggests the Mets' doctors were confident that the cause of his discomfort was not serious. But such a test would be necessary if Santana's problems were to persist.

"It can be just overuse," said Dr. Jonathan Glashow, co-chief of sports medicine at Mount Sinai Medical Center. "But if he stays out for a longer period of time, or if this keeps coming up, it could point to something structural."

The Mets have a significant investment in Santana. He's not only earning $22.5 million this year but is owed another guaranteed $55 million through the 2013 season. There is a $25-million club option for 2014 that could switch to Santana based on vesting terms.

General manager Sandy Alderson stressed that Santana's health for 2012 is the primary goal and said a return this season is not a critical part of that process. Santana had surgery Sept. 14 to repair a torn anterior capsule in his shoulder, a non-arthroscopic procedure that can result in a lengthy rehab. Former Yankee Chien-Ming Wang had a similar surgery in 2009, and his first post-op start was July 29 -- two years to the day after his operation.

"It's a matter of Johan proceeding at his own pace, and he's going to have to be sensitive to how his body responds to the increased workload," Alderson said earlier this week. "This is -- I wouldn't say it's something that we expected to happen. But during the overall course of his rehab, it's probably not something that should be unexpected."

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Santana made one rehab start July 28 for Class A St. Lucie. He pitched three scoreless innings, allowed two hits and struck out three.

After that outing, he did his usual between-starts regimen, throwing a side session Saturday and playing long toss Monday with the expectation he would start Wednesday or Thursday. But that never happened. Now the Mets have halted his 30-day rehab clock, making a new return date impossible to predict.