"It's an issue," Amaro said before Tuesday's exhibition game against the Pirates at Citizens Bank Park. "Obviously, losing your closer for this amount of time is not something you're looking forward to. We knew he had something going on in there."
Lidge, who will start the season on the DL for the third time in the last four years, can begin throwing in three weeks.
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If everything works out as planned, Lidge is likely to return to the active roster by the end of May, at the earliest.
"On the brighter side, it's not something that needs to be surgically repaired," Amaro said.
Lidge visited team physician Michael Ciccotti on Friday, and followed that up with Tuesday's MRI.
The Phillies were concerned with Lidge's velocity this spring, which was clearly down from last season. He was sidelined for two weeks earlier this month with biceps tendinitis in his right arm.
Lidge was not sharp in his last Grapefruit League outing, when he gave up two hits, two walks and a home run in two-thirds of an inning against the Twins on March 24.
"I haven't had a chance to talk to Brad, but I'm sure he's not happy with the news," Amaro said.
Phillies manager Charlie Manuel said that Jose Contreras would currently be his choice as the club's closer. Ryan Madson is also an option.
"If the season started today, my number one choice would be Contreras," Manuel said.
Lidge was 1-1 with a 2.96 ERA and 27 saves in 32 opportunities last season.
Andy Jasner is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.