If you’re anything like me, you kind of forgot that Zach Duke was still a major-league baseball player. And in fairness, less than a year ago, Duke no longer looked like a major-league baseball player. So the news that Duke got picked up by the White Sox for three years and $15 million came as a mild surprise to me, for two reasons. Immediately, I thought, that seems like a lot for a regular lefty middle reliever.

But Duke is no regular lefty middle reliever. In an effort to stick, Duke changed his throwing motion, which you can visualize here with the help of Texas Leaguers:

This past season, for a full season, Duke committed to a couple of arm slots, one lower and one higher. In 2013, Duke experimented with the lower slot against lefties, but in 2014 he used it against lefties and righties alike. For an idea of how this looks, behold 2011 Duke and 2014 Duke:

Duke, in addition to changing his delivery, has also tweaked his pitches and pitch mixes. The end result: Zach Duke, in 2014, in no way resembled previous editions of himself.

Let’s go back to 1950. Let’s look at all the pitchers who threw at least 30 innings in back-to-back seasons. All right, so, in 2013, Duke struck out just under 13% of hitters. In 2014, he struck out just over 31% of hitters. That gain of more than 18 percentage points is the third-largest gain in the spreadsheet. The second belongs to Wade Davis, and that can be explained by a shift from the rotation to the bullpen. The biggest gain, kind of hilariously: +21%, for 1974-75 Skip Lockwood. It’s hilarious because the biggest drop in the spreadsheet is: -20%, for 1979-80 Skip Lockwood. Anyway, enough about Skip Lockwood.

Zach Duke turned himself into something else. And though he lowered his arm slot, he was highly effective against lefties and righties, and he generated a ton of groundballs. Zach Duke became an awesome reliever, and with that in mind, consider this passage from MLB Trade Rumors:

Earlier today, reports indicated that David Robertson is seeking a four-year deal in the mold of Jonathan Papelbon‘s $50MM contract, and the asking price on top lefty reliever Andrew Miller might not be much lower. Jason Mastrodonato of MassLive.com hears from a source that Miller is seeking “at least” a four-year deal and isn’t listening to any offers that have anything less than an “astounding” average annual value.

Now is when I get to take some liberties. We know that, for the overwhelming majority of players, it’s important to look at more than just the most recent season. But with a guy like Duke, who’s changed his own profile, the past is of little predictive meaning, because what Duke did in 2012 doesn’t tell us anything of value about how he’ll do in 2015. He doesn’t throw like that anymore. So now, this table. Understand that this table is biased. It’s comparing one guy’s single season to two other guys’ past three seasons. But I still think this is of interest, even though I concede I’m pushing an agenda:

Pitcher	Year(s)	K% – BB%	FIP-	xFIP-
Zach Duke	2014	24%	56	56
Andrew Miller	2012 – 2014	27%	57	59
David Robertson	2012 – 2014	25%	64	62

Consider what you might expect Miller or Robertson to do in 2015. Duke kind of just did that. It was the first time he’d ever done that, but it was also the first time he’d ever pitched like he pitched, so. Miller and Robertson might be slightly more reliable, but the White Sox didn’t sign Duke for a Miller/Robertson price — they got Duke for a relative bargain, even though his odds might be almost as good.

So it’s an interesting move. The White Sox aren’t yet a contending team, so maybe it’s a little odd to see them putting a veteran in the bullpen on a multi-year contract. But for one thing, the Sox might not be too far away, and for another, if Duke does well, he becomes easily tradeable. If he has another season or two like the last one, he could bring back one or two good prospects, because he’d look a lot like how Andrew Miller looked. And if Duke busts, or gets hurt, or just under-performs, it’s not a crippling investment and the White Sox’s window isn’t yet open anyway. They can afford a wasted $15 million, if it comes to that. And the upside is significant.

Relievers are volatile, and Duke has had just one good season. Andrew Miller and David Robertson are also relievers. Maybe, the White Sox signed Andrew Miller lite. Or, maybe, the White Sox signed Andrew Miller.