Breakout season from Wilson increases value

As expected, the Tigers are officially open for business, opening trade season by sending outfielder J.D. Martinez to the D-backs for three infield prospects.
While you'll hear chatter about Justin Verlander or even Miguel Cabrera, neither franchise icon is the most likely Detroit player to be packing his bags next. It's lefty reliever Justin Wilson, and since he's not a rental and he should receive a reasonable rate in his final year of arbitration in 2018, he ought to bring back a lot more than Martinez did.
With nearly every contender -- the Dodgers, Cubs, Rays, Astros and Red Sox probably most prominently among them -- interested in lefty relief help, the Tigers will have no problem finding a market. And despite the fact that other lefties are out there, Wilson may be the best combination of talent and cost. Want San Diego's Brad Hand? His Andrew Miller profile and low salary will cost you a ton. Baltimore's Zach Britton? Prepare to empty your farm system. Pittsburgh's Tony Watson? He won't cost as much, but he's not nearly as good as Wilson, either.
Yet Wilson remains relatively anonymous, at least among the upper-echelon relievers in the game. He's probably not among the first 30 names you'd mention if asked to rank baseball's best relief arms; he might not be among the top 50. When the Tigers get something good for Wilson, many will be stunned. Let's go right ahead and change that. Wilson is better than you think. Here's why.

He ranks with baseball's best this year
The Wilson we saw from 2013-16, first with the Pirates and Yankees before joining the Tigers last season, was always pretty good. He just wasn't a standout. Over those four years, 224 relievers threw 100 innings, and Wilson was 100th in ERA (3.30) and 92nd in strikeout rate (24 percent). He was a perfectly solid, usable reliever, good enough to be traded both for Francisco Cervelli and then later the pairing of Luis Cessa and Chad Green.
Flash-forward to this year, half of which Wilson has spent as Detroit's closer after taking over from Francisco Rodríguez in May. Among 152 relievers with 30 innings thrown, Wilson's strikeout rate of 36.5 percent ranks 12th, ahead of Greg Holland and Wade Davis. That's such a big jump -- more than 10 percentage points -- that only five other relievers have eclipsed it this year, including All-Stars Craig Kimbrel and Corey Knebel.
That's pretty good. We can do better. For Wilson, it's not just about missing bats, it's about limiting dangerous contact. Using Statcast™, we can look at the combination of exit velocity and launch angle on every batted ball to see how often that ball turns into a hit. That way, we can do things like credit Wilson for inducing a very weak fly ball with a mere seven percent hit probability on May 24 ... even though Houston's ballpark turned it into one of the weakest homers of the year. It works the other way, too; when Wilson got Brett Gardner to line out last year, he really gave up a batted ball that's a hit 92 percent of the time, but Nick Castellanos was there to save him.

By crediting pitchers for the quality of contact they induce and the amount of contact they prevent, you get a much better understanding of the skill they're showing, independent from defense, ballpark, etc. When expressed as Expected wOBA -- which is just like OBP, except more credit is given for homers and extra base hits rather than treating each time on base equally -- you're left with the best understanding of what a pitcher is doing.
Last year, Wilson's xwOBA of .265 was 32nd of 241 relievers who had faced at least 100 hitters. This year? It's ... just a bit better.
Lowest xwOBA by a reliever in 2017 (minimum 100 batters faced)
.181 -- Kenley Jansen, Dodgers
.210 -- Craig Kimbrel, Red Sox
.212 -- Pat Neshek, Phillies
.213 -- Roberto Osuna, Blue Jays
.216 -- Felipe Vázquez, Pirates
.228 -- Andrew Miller, Indians
.229 -- Danny Barnes, Blue Jays
.230 -- Justin Wilson, Tigers
That's five All-Stars, including the unquestioned three best relievers in the game, plus arguably the biggest pitching snub in Rivero, and an unexpected breakout in Barnes, before Wilson. Remember, this includes strikeouts and inducing poor contact. Wilson has been one of the eight best so far.
That explains how good he's been. It doesn't explain what changed. We can do that too.

What's different this year?
It's no secret that Wilson is doing something different. He was pretty up front about it last offseason.
"Everything's hard, my breaking ball is hard," Wilson told MLB.com in January. "That's been a goal this offseason to get some separation there; mainly a true slider rather than a cutter that's trying to be a slider, or a bad curveball."
So that's exactly what he did. Last year, Wilson primarily threw three pitches: a four-seamer just over 40 percent of the time, plus a sinker and a cutter about a quarter of the time apiece. (He filled in the remainder with some rarely-used sliders, curves and changeups.) You'll notice all three of those pitches were types of fastballs. "Everything's hard," as Wilson said.
This year, the sinker is gone, and Wilson's reputation as a ground-ball pitcher is gone with it. Instead, the four-seamer is used 70 percent of the time. The cutter remains, but as he said, about 10 percent of his pitches are sliders. It's been useful; only one extra base hit has come off of it.
But what's really happened here is that throwing more four-seam fastballs is a very good thing, because not only is it thrown hard -- 96.1 mph on average -- it's got very high spin. Of the 201 relievers who have thrown at least 100 four-seamers, Wilson's spin rate of 2,453 rpm is 20th, and well above the league average of 2,276 rpm. High spin allows a fastball to defy gravity and sink more slowly, creating the "rising fastball effect," and it's positively correlated with swinging strikes and popups.
For Wilson, it's easily his best pitch. His swinging-strike rate on the four-seamer is elite, the seventh highest of any reliever, and he's allowed just a .101 average on it -- the best of any regular lefty reliever. It's not complicated: Wilson's four-seamer is very good, and he's throwing it more. He's long been a very good reliever who's turned himself into a near-elite option.
And since Wilson is not a rental -- he's making only $2.7 million this year, and he would likely get only a minor raise for 2018 -- he'd fit on any roster or budget. We know the Tigers are dealing. We know plenty of teams want a lefty. This one won't come cheaply.