Bobby Parnell returned from Tommy John surgery during the 2015 season, and the results weren't pretty. Parnell underwent the surgery in early April of 2014 and started his minor league rehab assignment one year and a few days later with the High-A St. Lucie Mets. He had a 10.80 ERA in his brief stint there and moved to Double-A Binghamton, where he had a 12.27 ERA.
Despite those results, the Mets called him up to the big leagues at the end of his rehab assignment in early June. Right from the start, Terry Collins used Parnell in high-leverage situations. In June, he was second only to Jeurys Familia in Fangraphs' gmLI metric, which measures a pitcher's average leverage when he enters the game. The results went well enough in the early going, as Parnell had a 0.73 through his first 12.1 innings.
Three outings in late July changed the tone of Parnell's season entirely. On July 22, the Mets had a 3-1 lead on the Nationals in Washington, and Parnell was called upon to handle the eighth inning and gave up three runs to give the Nationals a one-run lead that decided the game. In his next appearance, he gave up one run in the inning he pitched, though that wasn't an important outing by any means. And in the one after that, on July 30, Parnell gave up three runs in one-third of an inning against the Padres, turning a laugher in the Mets' favor into a close game. That was the nightmare-fueling game in which Justin Upton hit a go-ahead home run against Jeurys Familia after a very long rain delay—and immediately before another rain delay.
At the end of that outing, Parnell had a 4.91 ERA. By mid-August, it seemed very apparent that Parnell should not be pitching in high-leverage situations, and by gmLI, he typically didn't pitch in big spots the rest of the way. By the end of the season, Parnell had a 6.38 ERA and 4.18 FIP in 24 innings, and he did not make the team's postseason roster.
Parnell remains unsigned as spring training is set to begin around baseball. There hasn't been any indication that there's any chance he'll return to the Mets' organization. Assuming nothing changes in that regard, it's a shame that his Mets career will have ended so poorly. Before his surgery, Parnell was a very good—and seemingly very underrated—relief pitcher. Between 2012 and 2013, Parnell's 2.35 ERA ranked 28th among the 242 relief pitchers who threw at least 50 innings. That didn't put him up among the very best in the game, but he was somewhere in the next-best tier.
According to Brooks Baseball, Parnell's velocity last year wasn't terrible, but it was down a bit from where it had been before his surgery. Perhaps with the additional recovery time he's had this offseason, he'll bounce back and look more like his old self moving forward.