Will Andrew Miller succeed with the Yankees?

Relief pitchers are like running backs: The best ones don't remain at the top for long (with a few exceptions), as injuries and decline constantly refresh their ranks. For this reason, they generally don't receive long-term deals — teams just can't trust them to sustain their performance for the duration of the commitment. With that said, some relief pitchers are better than others. For these select few, things will always come easily; their employers compensate them accordingly.

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In the hopes that Andrew Miller falls into the latter group, the Yankees today agreed to a four-year, $36 million deal with the southpaw. How will we view the contract at this time in 2018? Let's look at the data to try and find out.

First, a bit of background on the man in question. The Tigers selected Miller sixth overall in the 2006 draft, and while the early results (125 ERA- in 76.1 major-league innings) certainly didn't impress them, they knew he would eventually become a top-of-the-rotation starter. The Marlins did too, so they requested him in the 2007 Miguel Cabrera trade, to which Detroit acquiesced. In Miami, however, he failed to realize his potential: Posting a 139 ERA- in 220.0 innings, he found himself traded to the Red Sox after the 2010 season.

Miller's first year in Boston saw him pitch at the same level as before, with a 130 ERA- in 65.0 innings. After that, though, he turned a corner — his park-adjusted ERA went from 79 to 64 to 51. His advanced stats mirror the improvement, as his xFIP- dropped from 81 to 62 to 42 in that same span. Like many failed starters before him, he had recaptured his magic (i.e., his velocity) by moving to the bullpen:

In particular, Miller's slider gained a lot of bite. With that added velocity came more whiffs, escalating from a 14.5% clip in 2012 to 20.8% in 2013 and 26.2% in 2014. Together with a 57.8% ground ball rate, the latter mark made the pitch worth 3.52 runs above average; among qualified AL relievers, only Wade Davis could do better.

But throwing hard doesn't necessarily equal prosperity. After all, Miller possessed formidable velocity in his early days (albeit not to this degree), and he didn't do well then. No, his change came with the absence of balls, which had held him back for the first several years of his career. Once he began to accrue more swings on pitches outside the strike zone — his O-Swing% jumped above 30% in 2013, and stayed there for 2014 — his walks vanished as well. Whereas prior to this year, he had walked 12.6% of the batters he faced, he lowered that figure to 7.0% in 2014. This accounted for most of his meteoric rise to stardom (although a 42.6% strikeout rate helps too).

Most breakouts bring some skepticism, what with random variation and all. In the case of Miller, though, any fears regarding his control are mostly unfounded. As his 7.8% xBB% proves, 2014 was no fluke. Steamer certainly agrees, foreseeing a modest 8.3% free pass percentage in 2015. That'll pair with a still-awesome 34.7% fan percentage to give him a 2.29 ERA and 2.29 FIP.

Despite all of that, one of Miller's drawbacks stands out: his age — he'll turn 30 in May. That's where we find the main problem with this deal; for when a player ages, his skill set inevitably diminishes, and that generally doesn't end well. To illustrate the extent to which this occurs, I've borrowed a graph from the distinguished work of BtBS alumni Bill Petti and Jeff Zimmerman. Behold:

Yeah, it's not pretty. This gets back to the original point about relievers: They break. Will Miller conform to this trend?

The evidence we have gives conflicting perspectives. On the one hand, research by Zimmerman has linked heavy slider usage to injury. As a hurler who derives his dominance from that pitch, Miller is probably at risk. On the other hand, research by...Zimmerman (damn, he just doesn't stop!) has linked good command to clean health. Should Miller maintain his newfound control, that would work in his favor.

Overall, the future looks kind of murky. Miller took a legitimate step forward this year, such that he shouldn't regress any time soon. However, he won't get any younger, and his maturity might do him in. In the end, it comes down to your preference: Do you want to go for it all, or do you like to play it safe? If you choose the former, then the deal checks out; if the latter, it probably doesn't. Because I belong to the first group, I think it represents a solid gamble for the Yankees, one that they should make as a win-now team. It has as much downside as any reliever contract, but enough upside to compensate for it.

The Yankees won't need Miller to be elite to the extent that some of his other suitors would have, since they have a homegrown relief ace in Dellin Betances. (Plus, they could still bring back David Robertson.) They'd obviously prefer he stayed phenomenal, though, and If he stays on the mound, that'll probably happen. We can't really evaluate these kinds of deals well in the moment, but this one looks fine to me.

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All data courtesy of FanGraphs and Brooks Baseball.

Ryan Romano is an editor for Beyond the Box Score. He also writes about the Orioles on Camden Depot and on Camden Chat that one time. Follow him on Twitter at @triple_r_ if you enjoy angry tweets about Maryland sports.