Jeremy Hefner never got a shot in the big leagues until the Mets called him up last year in his age-26 season. Since then, he's made 43 appearances, 29 of which have been starts, with a 4.32 ERA and 3.99 FIP. Working almost exclusively as a starter this year, he has a 3.54 ERA 4.31 FIP with decent rates of 6.8 K/9 and 2.6 BB/9.
With a young crop of arms in the minors, Hefner may very well be available if he's of interest to other organizations over the next few weeks. He's not arbitration-eligible until 2016 and is under team control through 2018. The Mets would have to plug someone into the rotation if they trade him away, but that's not likely a big concern over the course of the rest of this season.
The Blue Jays, Orioles, Giants, Indians, Rockies, Rays, and Diamondbacks are all in the hunt and could use pitching help. Hefner's track record is very short, but he's been a good pitcher. If the Orioles were willing to make a deal for Scott Feldman, whose contract expires after this season, one of these organizations might want to slot Hefner into the back of their rotation at an affordable price.
In order for the Mets to give up a potentially valuable asset like Hefner, they'd have to get a fairly big return. It's hard to imagine another organization giving up top-end minor league talent to acquire Hefner, but stranger things have happened in July. It's possible that Hefner is worth more to the Mets than he would be to another team, especially if the organization's budget for player salary will continue to be limited over the next few years. But if a good return is on the table, trading away Hefner is less risky proposition than sending Noah Syndergaard packing.