Between 2012 and 2014, Shawn Kelley was a tantalizing middle reliever. He consistently posted gaudy strikeout totals—with 11.02 per nine innings over that three-year span—but had mediocre control at 3.49 walks per nine and a tendency to give up home runs. The home run tendency was likely affected by pitching half his games at Yankee Stadium in 2013 and 2014. Some particularly bad struggles with men on base in 2014—opponents had a .240 wOBA against him with the bases empty versus a .345 wOBA with men on—led to a 4.53 ERA despite a 3.02 FIP.
After that rough 2014 season, the Yankees traded Kelley to the Padres for Johnny Barbato, In San Diego, Kelley enjoyed his finest season to date. He maintained his strong strikeout numbers with a 11.05 K/9 and posted a vastly improved 2.63 BB/9 and 0.70 HR/9. His improved peripherals led to a career best 2.45 ERA and 2.57 FIP.
Further, he induced more ground balls than fly balls for the first time in his career, flipping from a 0.69 career GB/FB to 1.13. With his improved control and new-found proclivity for ground balls keeping batted balls out of the seats, Kelley saw the best results of his career.
There are some caveats, though. Kelley is 31 years old, and this is only a 51.1-inning sample. For his career, Kelley actually has reverse platoon splits, but those numbers flipped in 2015, which could be a statistical fluke. He also missed two weeks towards the end of the season with a forearm injury, which is concerning since he has had Tommy John surgery twice. He did return to pitch five scoreless appearances late in the season, but there's still some concern there.
Fangraphs' Dave Cameron predicts a three-year, $18 million contract for Kelley, while MLB Trade Rumors has Kelley receiving two years at the same AAV. While non-elite middle relievers are essentially a random number generator, the Mets need bullpen reinforcements, and a free agent at this price range should be affordable even with the club's financial limitations.