The Yankees bullpen is short on neither talent nor name value. Most baseball fans are aware of Aroldis Chapman, even if they’re not aware that he’s been in a funk. Likewise, many baseball fans are aware of Dellin Betances, and they’ve generally heard of David Robertson, who this year has re-emerged as terrific. To make the unit all the more deep, the Yankees acquired Tommy Kahnle, one of the year’s big bullpen breakout stories. Kahnle has struck out 38% of his opponents, ranking him seventh in baseball among regular or semi-regular relievers.

Kahnle, in the breakout sense, is a surprise. He finally has the results to go with the powerful stuff. And yet Kahnle may play second fiddle in here, because of another development. The highest strikeout rate for any reliever belongs to Craig Kimbrel. Well, sure. In second place, we find Kenley Jansen. Yes, of course. The name in third place reads “Chad Green.” And he’s up around 60 innings, so this isn’t just a hot week or month. Green has piled up the strikeouts, and as a consequence, he ranks seventh among all relievers in WAR. Chad Green made his season debut on May 9.

Green appeared in 12 major-league games last season, and he was fine. For eight of the 12 games, he was a starter, and if you look at this season, he’s been the starter in just one of 32 games. Right there, you might begin to understand what’s allowed Green to take off. He wouldn’t be the first mediocre starter to find a welcome home in the bullpen. Absolutely, the shift has helped, and David Laurila recently caught up with Green in the clubhouse. Green mentioned that he’s been able to simplify his repertoire by narrowing down.

Yet this isn’t only about stuff playing up in short outings. For one thing, Green is tied for third in baseball in number of relief appearances lasting more than one inning. He’s been more of a multi-inning weapon, which places greater demands upon the arm. And Green, simply, is better. He has better command of his stuff, and he’s turned into something rather overpowering.

As I’ve mentioned in the past, one of my preferred measures of dominance is contact rate within the strike zone. Anyone can get a hitter to miss a slider low off the plate; it takes something more to get a whiff in the hitting area. In the following plot, you’ll see in-zone contact rate for this season, as well as general zone rate. Green is the point highlighted in yellow.

Two things going on here: Green has missed plenty of bats in the zone, and Green has also thrown plenty of pitches in the zone. The latter would follow from the former, I suppose — no need to try to get hitters to expand when you can make them miss over the plate. This is a fairly ideal combination of traits, and if that in-zone contact rate strikes you as being extremely low, well, it is! Here are the lowest rates over the past decade.

Lowest In-Zone Contact%, 2008 – 2017 Pitcher Year Z-Contact% Aroldis Chapman 2014 63.2% Aroldis Chapman 2015 67.1% Kenley Jansen 2011 67.3% Aroldis Chapman 2012 67.4% Craig Kimbrel 2017 69.3% Craig Kimbrel 2012 70.3% Ernesto Frieri 2012 70.6% Kenley Jansen 2017 71.7% Chad Green 2017 71.8% Seung Hwan Oh 2016 71.9% Minimum 50 innings thrown.

There’s Green, in ninth, hanging out with some elites. Hanging out also with Ernesto Frieri, but even he had his seasons. How does a pitcher miss so many bats with would-be strikes? The answer almost always involves some kind of overwhelming or deceptive fastball, and, sure enough, Green’s fastball has become something extraordinary. I looked at every pitcher-season with at least 500 four-seam fastballs thrown since 2008. Here are the lowest contact rates on said fastballs.

Unhittable Four-Seamers, 2008 – 2017 Pitcher Year Fastballs Contact% Aroldis Chapman 2014 639 58.1% Aroldis Chapman 2015 875 59.7% Chad Green 2017 632 59.8% Craig Kimbrel 2017 662 60.0% Vinnie Pestano 2011 652 60.9% Craig Kimbrel 2012 619 61.7% Brad Boxberger 2014 681 62.0% Aroldis Chapman 2012 1058 62.4% Takashi Saito 2010 550 62.9% Aroldis Chapman 2013 937 64.1% SOURCE: Brooks Baseball Minimum 500 four-seam fastballs thrown.

Chad Green, behind only Aroldis Chapman. With a sitting velocity around 96, Green isn’t up there throwing slop or anything, but he doesn’t quite have Chapman velocity. Yet the results are the results, and, as Green said to Laurila, part of this breakout is about relying on the four-seamer more often. That’s easy enough to observe through Green’s admittedly limited major-league history.

As Green has thrown more four-seamers, at the expense of everything else, he’s collected more and more strikeouts, as opponents haven’t yet come up with any kind of consistent solution.

As a reliever, Green has added a little bit of zip. He’s also been able to focus on his four-seamer and breaking ball, moving away from sinkers, cutters, and changeups. If you talk to Green, as Laurila did, he’ll talk about the importance of that breaking ball, to give the hitter some kind of other look. Without question, Green’s better this way than he would be if he just threw four-seam fastballs 100% of the time. But the real move here, to me, seems to be improved fastball command. Here come some Baseball Savant heat maps. You see 2016 and 2017 four-seam fastballs, against righties and against lefties.

Against righties, Green looks somewhat similar. He’s right-handed, himself, and he’s located a little better up and to the glove-side. But then look at Green below, against lefties. A season ago, his four-seam fastball was a pitch he tried to keep over or around the inner half. This year, he’s completely changed sides, attacking arm-side. To review, then: Against righties, Green has spotted his fastball on the glove-side edge, and then against lefties, he’s stayed around the arm-side edge. The command improvement is real, and to both sides. The result is that Green gets plenty of swings like this.

There’s not really anything else that Green needs, not as long as he’s a reliever. He has something of a high-spin rising four-seam fastball, and he’s learned how to keep it elevated. He’s now able to command the pitch to both sides, and he’s certainly not hurting for any speed. There’s a breaking ball in there to keep hitters honest, and Green is consistently good about keeping it down, where it ought to be. The breaking ball will be key, because without it, Green could conceivably go the way of Frieri before him, where home runs spiral out of control. Green will need more than *just* the fastball, most of the time. But the fastball is also the primary tool, and it’s a dominant one. With an overpowering fastball and an adequate second pitch, Chad Green has become one of the better relief weapons around. The Yankees, as a consequence, have an embarrassment of bullpen riches, and for that reason more than any other, they could be a nightmare of an October opponent.