Whether or not you really value ERA as the best tool to evaluate a reliever -- there's certainly plenty of reasons not to, and no one is really arguing that the Yankees, for example, would rather have Kelley back than Chapman, Miller or Betances -- the greater point of the unheralded Kelley being secretly dominating remains. In 19 appearances this year for Washington, he remains unscored upon, with a perfect 0.00 ERA.
But how "secret" his success really is depends on your perspective. Through the end of the 2015 season, Kelley had two Tommy John surgeries (2003, 10), been with three teams (Mariners, Yankees and Padres), and recorded four saves (all in '14 with the Yankees, subbing for an injured David Robertson), along with a reputation for fragility -- he missed time in 2014 with a back injury and in '15 with a calf injury and forearm soreness.
Yet as Kelley worked around those setbacks, he piled up the strikeouts, 63 in 51 1/3 innings in his lone season with San Diego. Last winter, before he signed a three-year deal with the Nationals, we looked at value free agents and compared Kelley's previous three seasons favorably with that of Cardinals closer Trevor Rosenthal -- they shared nearly identical stats in strikeout percentage (30.4 percent), walk rate (approximately 9 percent) and batting average against (.224/.225). Rosenthal throws harder and pitches in the ninth, so his national profile was immeasurably larger.
So the fact that Kelley has had under-the-radar success over the past few years, when healthy, somewhat mitigates the "how is he doing this all of a sudden?" question, because it's not really all of a sudden. Since 2013, 328 pitchers have thrown at least 150 innings, and only 11 have had a higher strikeout rate. Still, this year is another step ahead, because Kelley has pulled off a neat trick: He's increased his whiff rate from 30 percent to nearly 38 percent, and he's generating plenty of popups.
Popups, remember, are basically strikeouts. Last year, the Major League average on popups was .021, which basically means the hitter is doomed unless the fielders put together a blooper reel. Other than strikeouts, it's basically the best thing a pitcher can do. Remember how well Kelley is doing in whiffs, and now realize where he ranks in inducing pop-ups:
Kelley hasn't really changed his pitch mixture, still using the slider slightly more than half the time and his four-seam fastball slightly less than half the time. He's not throwing harder; if anything, he's down a half a tick, not unexpected given the chilly spring in most of the Northeast.
So what is different for Kelley? It's not the slider, even though that remains his best weapon and he's throwing it in the zone a bit more. Last year, Kelley allowed a .169 batting average on the slider; this year, .152. Last year, he had a 40 percent strikeout rate on the slider; this year, 42.4 percent. The slider, as good as it is, was always good.
But the fastball has changed. Last year, Kelley allowed a .300/.329/.414 line against it; and this year, that's just .238/.273/.333. If you look at the difference in fastball placement, the difference is subtle, but real:
You can see that last year, Kelley's fastballs were mostly right down the pipe, middle-middle. This year, he's still throwing strikes -- his overall zone percentage hasn't changed -- but he's moving it around more, hitting better spots, and even elevating. Even though Kelley is throwing the same amount of strikes, he's throwing better strikes, and the contact rate on his has dropped from 80 percent to 70 percent. If you look at Edge Percentage, which measures the pitches that were within 3 inches either way of the side of the plate, Kelley's percentage of pitches "on the black" has gone from 23.6 percent to 27.5 percent. That's the difference between strikes, and good strikes.
When Kelley uses them together, they can be deadly. Giancarlo Stanton found that out this year, striking out on each, even if these are from two games. Out of the hand, it's difficult to know if it will dive or stay up.
So Kelley has usually been good, and now he's been even better. Other than "can he stay healthy," the only question left at this point is, "when will he usurp Jonathan Papelbon in the ninth inning?" Papelbon's velocity and strikeout percentage have been on a steady decline for the past five years, and it's easy to argue he's no longer the best pitcher in the Nationals' bullpen.
Of course, it's not hard to say that Kelley is Washington's best reliever. Over the past year, he has been something like the sport's best reliever, even if the lack of saves has helped to obscure that. So long as Kelley keeps this up, it won't matter when he pitches. He'll get plenty of attention.
Mike Petriello is an analyst for MLB.com and the host of the Statcast podcast. This story was not subject to the approval of Major League Baseball or its clubs.