When the Yankees traded Francisco Cervelli to the Pirates for reliever Justin Wilson, a lot of people asked: "What is a Justin Wilson?" Wilson wasn't by any means well known around the majors, and few people could agree on what the Yankees were getting in terms of talent - yes, he had a power arm, but he also had a penchant for walks. Few probably expected him to be the third best pitcher in the bullpen coming into the season, but sitting here in October, he's clearly the one Girardi trusts, and with good reason, to bridge the gap between starters and the eighth and ninth innings.
Wilson's posted his best fWAR ever this season, coming in at 1.4 - good enough to be top 20 among relievers. He's done it by increasing his strikeouts to their best mark ever (9.64 K/9), while limiting the walks that so many people worried would cripple his effectiveness. In just under 61 innings this season, Wilson has walked just 2.97 batters per nine innings, by far the best mark of his career and well under his 4.50 rate from last season. Harnessing his fastball and improving his control has done a lot to make Wilson so effective, and it's made him one of the best non-closer relievers in all of baseball.
Wilson's ability to limit home runs has also helped him this year. This can be huge for relievers, since they often find themselves coming in with runners on base. He's posted a 0.45 HR/9 rate along with giving up homers on just 6.8% of all the fly balls he's given up, which lead the Yankees bullpen mainstays. This has long been a strength of Wilson's, as his career HR/9 mark is just 0.50, but his continued ability to keep balls on the ground and out of the seats has been even more valuable playing half his games at Yankee Stadium.
As far as his pitches are concerned, Wilson mainly throws a four-seam fastball (over 50% of the time), mixing in a sinker and cutter, and occasionally letting a slider sneak in there. His four seamer is his main weapon, sitting around 96 mph, and, with some of the most vertical movement in the game, seems to rise on hitters quite a bit. This makes it hard to handle, as opposing batters have hit just .192 with a .240 slugging against his fastball this season. His sinker's terrific at generating ground balls (75% grounders on balls put in play), and also rarely gets hit for a line drive. The pitch he's had difficulty with this season is his cutter, which is one of the pitches the Yankees liked best of all when they acquired him. It's generating fewer whiffs per swing than his sinker and four-seamer, and is also hit more frequently for line drives and homers than those pitches. Correspondingly, hitters have managed to bat .303 with a .546 slugging percentage off that pitch. If he can fix whatever seems to be working against him with his cutter, Wilson could be even more dangerous than he currently is.
Justin Wilson seemed like one of the smaller acquisitions the Yankees made this offseason, but as the season wears on and the bullpen seems thinner and thinner, he may end up being one of the most important. His four-seamer's been as advertised, and his ability to up his strikeouts while finally sorting out some of his control issues has helped him unleash his potential. The best part - he's under team control for three more seasons after this one. Wilson looks like an integral, and effective part of the bullpen for the foreseeable future.