Matt Harvey didn't pull any punches about how he felt his slider had worked (or not) in 2015, recently telling MLB.com's Anthony DiComo that he "just couldn't really throw it last year until the end," and that he "really didn't have it for, I'd say, three-quarters of the way." In further comments, Harvey lamented the ineffective spin he felt he had on the pitch for much of the season.
Considering that Harvey -- who made his Spring Training debut on Tuesday against the Braves -- had missed more than a full season recovering from Tommy John surgery, it wouldn't be unexpected that he needed some time to work out the kinks. And since his slider was arguably his most feared weapon before surgery -- in 2013, it was his best whiff-per-swing pitch -- it'd be understandable if not having it at his disposal would have made his first season back below his standards.
But that's not what happened. Harvey set a career high in innings, put up a better ERA than Madison Bumgarner, Sonny Gray or Max Scherzer, won the National League Comeback Player of the Year Award and pitched effectively in the postseason. He was great in just about all the ways a pitcher can be great -- largely without, as he said, his most notable pitch.
So now we have a premise ready-made for Statcast™. Harvey believes his slider wasn't working for most of last season, and he believes that spin was a large part of the reason why, but that it turned around as the season went on. If he's correct, then we should already be able to find signs in the data that the pitch behaved different, and by extension, should be in his arsenal to start 2016.
Is Harvey correct? In a word, yes.
Harvey noted to DiComo that he wasn't sure if it was a "a forearm-strength thing or a feel thing," and in late July, it was reported that he'd made adjustments to the slider with pitching coach Dan Warthen. But the trend was unmistakable. Just look at what happens when you compare spin rate (in blue) to the vertical movement he gained (orange line):

It's a pretty big difference. In April, Harvey's slider spun at 2,083 rpm and had an average movement of 2.4 inches, vertically. In September, those numbers were up to 2,336 rpm and 4.9 inches, which is to say that he doubled his movement. Now, we didn't have Statcast™ in 2013, so we can't compare Harvey's spin rate, but we can look back at the pure movement, and back then, it was 4.0 inches, which says a lot about what he was doing by the end of the year.
If you look at 2015 as a whole, the pitchers with the highest vertical movement on their sliders include Jacob deGrom, Justin Verlander, Harvey and Jordan Zimmermann. Now, there's an argument to be made that high vertical movement isn't always what pitchers want, as that infers that the ball stays up, and Garrett Richards, who had the lowest vertical movement (also expressed as highest negative movement) allowed just a .194 average on his sliders. That said, we know that there are many different types of sliders, and the Mets in particular teach the "Warthen slider," which isn't necessarily intended to be like other sliders. Either way, anything that makes Harvey's pitch look more like it did in 2013 can hardly be a bad thing.
Gif: Matt Harvey strikeout reel
So what sort of effect did it have? Harvey's adjustments with Warthen came prior to a July 25 start against the Dodgers, so let's use that as the dividing line:
Harvey
Through July 24: 3.04 ERA / 3.62 FIP / 23.4 K% / 6.3 BB%
July 25 and after: 2.15 ERA / 2.11 FIP / 27.4 K% / 2.5 BB%
Not bad. Now, look at the behavior of the slider during that same period:
Slider
Through July 24: 42% swings / 9% fouls / 17% swing-and-miss
July 25 and after: 54% swings / 16% fouls / 20% swing-and-miss
Though it wasn't necessarily more dominant -- the batting average against rose slightly from .152 to .182 -- you can see how the behavior changed. Able to locate it more in the zone, making it look more appealing, Harvey induced more swings, more fouls (which are good, don't forget) and more misses.
You can't simply take Harvey's second-half performance and assume that's what he'll be all season. If you could, he'd have had just as much claim to the NL Cy Young Award as Jake Arrieta, Clayton Kershaw or Zack Greinke. But Harvey will be another year removed from surgery, and he's shown that his successful first year was great despite being something of a work in progress. If he's regained that slider, well, it just won't be fair. Not that it ever was, of course. Whenever you can reasonably make an argument that Harvey is the third-best starter on his own team, there's nothing fair about that.