The news for Mejia was expected, since he had been pitching through bone spurs and was already scheduled to have minor surgery on his elbow after the season. For Mejia, while this ends his season prematurely, having the surgery now (it’s been reported that he’ll have the surgery in 7 to 10 days) means that he’ll likely be good to go well before spring training.
For Hefner, the news is much worse. He was initially diagnosed with a strained MCL in his pitching elbow, and had hoped to return later this season. If he opts to undergo Tommy John Surgery, Hefner would likely miss most, if not all of the 2014 season. The expected recovery time after Tommy John Surgery is 12 to 18 months.
…as is noted above, while this is unfortunate for Mejia, the surgery he’s having in about a week was one that he was having sooner or later regardless. He should be fine.
In the case of Hefner, this is a blow for both he and the Mets. The team will likely go into spring training next year with Matt Harvey, Jonathon Niese, Zack Wheeler, Jenrry Mejia, and Dillon Gee as their five starters.
If any of those pitchers were to be traded or injured, Hefner would’ve likely stepped in. Even if he didn’t make the rotation, Hefner may have made the bullpen as a long man. If he undergoes Tommy John Surgery, both of those potential 2014 options will be out the window.