The Mets might have their explanation for Jeremy Hefner’s brutal second half. Hefner yesterday was diagnosed with a partially torn medial collateral ligament of the right elbow and placed on the disabled list retroactive to Saturday. Because of the DL move Hefner’s reassignment to Triple-A Las Vegas was nullified. The Mets are calling the ailment a “chronic” condition that worsened as the season progressed. Hefner has a 9.13 ERA in five appearances since the All-Star break after a dominant stretch to close the first half of the season. Manager Terry Collins said it’s possible Hefner will pitch again this season. The right-hander has been told to rest and will be re-evaluated. Collins said the diagnosis likely explains Hefner’s recent ineffectiveness. “This certainly had a lot to do with it” Collins said before the Mets’ 5-4 12-inning loss to the Dodgers last night. “I know he could pitch with it that’s why he did but we just saw a big difference that there had to be something that we tried to address.” Collins said the organization was aware of Hefner’s elbow soreness since at least June. “One of the things we talked about when we discussed a six-man [rotation] was there was so many other things involved” Collins said. “[Hefner] Jon Niese Zack [Wheeler]. It could have helped a lot of guys that extra day.”