Kirby Yates and WPA

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Win Probability Added is a statistic that credits and debits a player based on how their performance impacts their team’s chance to win a game. If a batter hits a walk-off home run that pushes his team’s win probability from 40 percent to 100 percent, that dude gets .60 WPA points. The pitcher, correspondingly, loses just as much off his ledger. That’s a big swing, of course. Most of the credits and debits are much smaller—a single here, a walk there, a fly out here.

While most baseball stats—like WAR or wRC+, for instance—are context neutral, meaning they’re designed to ignore the situation of the game in which a player’s contributions occur, WPA is just the opposite. It relishes the context, doling out points based on a player’s timeliness. That makes it a great little tool to use for late-inning relievers, since they often enter with the game on the line.

Last season, National League MVP Christian Yelich led all hitters with 6.02 WPA, and the AL MVP, Mookie Betts, finished third at 5.77. On the pitching side, Jacob deGrom’s sensational season was worth 5.85 wins by WPA, first among starters. But Blake Treinen, a reliever from Oakland, led all of baseball in WPA, netting 6.22 wins on the strength of an absurd 0.78 ERA in 80 1/3 innings.

By any version of WAR, Treinen was an afterthought. Per FanGraphs, Treinen’s 3.6 WAR ranked 22nd among pitchers, right between James Paxton and Kyle Hendricks. Good, not special. But by WPA, however, Treinen’s 6.22 mark ranked 14th among all pitchers since 1988, when the modern bullpen started taking shape. Only Pedro Martinez, Greg Maddux, Roger Clemens, Orel Hershiser, Zack Greinke, Randy Johnson, Keith Foulke, Troy Percival, Zack Britton, Eric Gagne, and Randy Johnson have bettered that 6.22 number over the last 31 years. For relievers, WPA is the equalizer, putting them on similar footing to position players and starters.

Nobody would take Treinen over Yelich, Betts, or deGrom, of course, but WPA illustrates just how big a role a shutdown reliever can play. Since Treinen was used almost exclusively in important situations, and since he hardly ever surrendered even a lowly run, he racked up a ton of value for the overperforming A’s in 2018. In fact, it’s easy to credit a decent-sized portion of the A’s out-of-nowhere success to Treinen himself. With a run-of-the-mill closer, last year’s A’s might have missed the playoffs. With him, they won 97 games and cruised to a wild card spot.

Here are the top 10 reliever seasons since 1988:

1. Keith Foulke2000White Sox6.62
2. Troy Percival1996Angels6.54
3. Zack Britton2016Orioles6.33
4. Eric Gagne2003Dodgers6.32
5. Blake Treinen2018A’s6.22
6. Trevor Hoffman1998Padres5.85
7. Jose Mesa1995Indians5.83
8. J.J. Putz2007Mariners5.82
9. Brad Lidge2004Astros5.81
10. Joe Nathan2004Twins5.77

The top 10 is conspicuously missing Yankee great Mariano Rivera (maybe his teams were too good), but it includes several all-time closers, like Trevor Hoffman, Gagne, and Foulke. Note how rare Treinen’s 2018 turned out to be, though. He’s one of just five relievers since 1988 to crack the six WPA threshold in a single season.

Enter the 2019 version of Kirby Yates. So far Yates is 20-for-20 in saves, with 12 of them the one-run variety. He’s allowed just three runs all season, two in back-to-back losses to the Dodgers and one with a 4–0 lead against the Diamondbacks. Yates has put up a 1.13 ERA, with 44 strikeouts, eight walks, and no homers allowed in 24 innings. His 46.8 percent strikeout rate is fourth in baseball, behind only Josh Hader, Mike Clevinger, and Matt Barnes. His K%-BB% is fourth to the same three guys; his 0.66 FIP is second to Clevinger; his cFIP is second to Barnes; his 2.29 DRA is ninth. You get the point.

By WPA, Yates is at 2.14 already, trailing only San Francisco’s Will Smith by .04. Smith and Yates are the only two pitchers with at least 2.0 WPA. To put into context just how good Yates has been in this category, consider that last season—when he pitched 63 innings and posted a 2.14 ERA—he accumulated just 2.57 WPA all year. And that number ranked 10th among relievers. Sooo, Yates has already had a good season for a reliever. It’s May 24.

It’s impossible for the baseball-loving fan not to extrapolate from here, if only for informational purposes. Yates has recorded his 2.14 WPA through 50 team games. If he keeps up this pace through the rest of the season, he’ll finish with 6.9 WPA, blowing away Foulke’s longstanding (modern bullpen era) record.  

Will it happen? Well, probably not. Baseball is designed to take the best of on-pace-for plans and shred them to pieces. Even though Cody Bellinger is suddenly a superstar, he’s almost certainly not going to rack up a Ruthian 13.2 WAR this season. Same thing with Yates. Chances are, he’ll give up a few more runs down the stretch, cough up a few big saves. Regression to the mean is omnipresent. Further, the Padres have afforded Yates with plenty of opportunities. One-run saves, specifically, are a gold mine for WPA. Since the game is tighter, there’s more WPA points available in a one-run save than the two- or three-run type. The Padres might not play such a high percentage of close games from here out.

But could it happen? Heck yeah. Yates and these Padres might just be the perfect match for some WPA magic. We already know that Yates isn’t getting by with much good future. Sure, he might surrender a couple home runs here or there, but he’s put up that 1.13 ERA with a .375 BABiP. When you strike out half your opponents, luck doesn’t play much of a role. Roughly speaking, he’s going to continue to dominate.

The question might come down to whether the Padres can continue feeding Yates so many save opportunities. His 20 saves pace baseball by five, with Detroit’s Shane Greene in a lone second. He also has four more save opportunities than the next closest guys, Greene and LA’s Kenley Jansen. Consider this: Yates has as many one-run saves than all but four other closers have total saves. Save chances can come in bunches or disappear for a week stretch, but the Padres—good but not too good—could be the right sort of team to keep supplying Yates with plenty of WPA ammo.

As it stands now, Yates is having one of the finest reliever seasons ever, by just about any metric. With over four months left in the season, there’s plenty of time for him to come back to earth some. There’s also enough time for him to chase down some records. With how well he’s pitched since joining the Padres, the latter scenario almost seems more probable.  

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