Sean Gilmartin's 2015 can be considered nothing but a success. As a rare Rule 5 draft pick that actually stuck on a major league roster throughout the season, Gilmartin gave the Mets 57.1 innings of 2.67 ERA ball and is now totally controlled by the Mets. Long or middle relievers are hardly the most sexy pieces on a ball club, but Gilmartin was a solid contributor for the Mets, and he even delivered a solid spot start in once the Mets had clinched the division.
When the Mets drafted Gilmartin, they expected to get a lefty specialist who could help handle some of the big left-handed bats in the division. The idea had merit, as lefties hit just .201/.219/.235 against him in the minors in 2014, and the Mets were desperate for left-handed relief at the time. Instead, Gilmartin actually posted reverse platoon splits, allowing a .294 wOBA against lefties and a .250 wOBA against righties, working mostly as a long reliever. A good portion of that difference came from batting average on balls in play—.343 for lefties versus .270 for righties—but his walk and strikeout rates were pretty similar against both sides of the plate.
In terms of his baseline production, Gilmartin probably over-performed a bit last year. His 2.75 FIP basically matches his ERA, but his 3.82 xFIP is a much less rosy projection. Gilmartin allowed home runs on just 3.4 percent of his fly balls last year, and some regression in that department should inflate his ERA a bit. Some of that gain could be offset by a lower BABIP—Gilmartin induces lots of pop-ups and ground balls while limiting line drives—but expecting an ERA below three again is probably optimistic. Projection systems call for an ERA in the mid-to-low-3s, which is probably more representative of Gilmartin's true talent.
GIlmartin has a sizable minor league track record of shutting down lefites, so it's too early to write him off as a potential lefty specialist just yet. For now, however, the Mets have no reason to push him into that role, particularly since they brought back Jerry Blevins. Gilmartin should slide back into the long reliever/spot starter role he held last year, though the Mets might be better served sending Gilmartin to Triple-A to stay stretched out as a starter in case of an injury. Either way, he should serve as a solid, if unspectacular, depth piece for the 2016 Mets, an excellent outcome for a Rule 5 pick.