The Mets had removed Hefner from the active roster during the weekend, when they needed to create a spot for the return of left-hander Jonathon Niese from the disabled list.
Initially, Hefner was optioned to the minors. But general manager Sandy Alderson said that demotion would be rescinded and Hefner instead would be placed on the DL if an examination with team doctors determined he was injured.
A follow-up exam Wednesday at the Hospital for Special Surgery in Manhattan determined Hefner has a partial tear of the MCL, the team announced. A team spokesman described it as a chronic condition that worsened.
Hefner will not require surgery. Manager Terry Collins indicated Hefner should return this season.
"Everything I've heard, there's a good possibility he'll be back," Collins said. "I think with some rest and some strengthening stuff he should be able to come back."
Hefner had been ineffective since the All-Star break, going 0-2 with a 9.13 ERA in five starts. So rather than go to a six-man rotation -- the original plan articulated by the organization with Niese returning from the disabled list -- the Mets decided to remove Hefner.
Hefner is 4-8 with a 4.34 ERA in 24 appearances (23 starts) this season.
"I know he could pitch with it. That's why he did," Collins said. "We just saw such a big difference, where there had to be something that we tried to address. He just said, 'Look, my arm just doesn't feel the same -- the release point and everything else.' It certainly is understandable."