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A’s bullpen is stacked entering 2021

There are no weak links in this group, according to the predictive stats.

Division Series - Oakland Athletics v Houston Astros - Game Four
Diekman didn’t allow a hit this spring in 6 innings
Photo by Harry How/Getty Images

The Oakland A’s had arguably the best bullpen in baseball in 2020, led by AL Reliever of the Year Liam Hendriks and All-MLB Team finalist Jake Diekman.

They lost Hendriks over the winter in free agency, as well as setup man Joakim Soria, and yet somehow came out on the other side with a potentially upgraded pen. Replacing Hendriks is Trevor Rosenthal, himself a former All-Star who nearly matched Hendriks’ lofty numbers in 2020, and in for Soria is Sergio Romo, who is basically an exact replacement in terms of production. On top of that, they enhanced one of the middle relief spots, from a mediocre innings-eater in T.J. McFarland to a tougher lefty coming off an 0.95 ERA last summer in Adam Kolarek.

But whether this year’s group is technically better or worse isn’t important. Rather, they just need to keep being good right now. And there’s promising news on that front.

We use all kinds of stats when measuring a reliever’s past performance, from ERA to strikeouts to saves to Win Probability Added. But at a position where year-to-year volatility is the law of the land, there are two mainstream metrics in particular that are useful for predicting relievers into the future.

One is FIP, which strips out many of the external factors in a pitcher’s results by looking at just strikeouts, walks, and homers, and ignoring things they can’t control like the defense behind them. It’s designed to read like an ERA, but instead of counting actual runs it defines success more theoretically by suggesting how a pitcher with those fielding-independent results would typically be expected to perform.

The other is xwOBA, courtesy of Statcast. Similar to FIP’s relation with ERA, xwOBA is a number that’s designed to read like wOBA, which itself is sort of a more advanced version of OPS but with values in the OBP range (around .300 to .400+). However, instead of using the actual box-score results like hits and homers, it uses raw batted ball data like exit velocities and launch angles. An opponent’s OPS will tell you how many of their batted balls fell, but their xwOBA will tell you how well they hit the ball and how that kind of contact is expected to turn out in general.

There are other fancier metrics out there, and then even more used by teams behind the scenes on a proprietary basis that we’ll never know about, but these two provide a big picture in terms of what to pin your hopes on for next year.

The A’s will have eight relievers on their Opening Day roster, and they all rank wonderfully on both of these scales. In fact, last year they were pretty much all better than league average.


2020 MLB reliever average: 4.45

  • Trevor Rosenthal: 2.22
  • Burch Smith: 2.36
  • Jake Diekman: 2.72
  • J.B. Wendelken: 3.07
  • Adam Kolarek: 3.14
  • Lou Trivino: 3.92
  • Yusmeiro Petit: 4.11
  • Serigo Romo: 4.34

All eight beat the league mark. Among 141 relievers with at least 20 innings last summer, Rosenthal ranked 13th in this department, with Diekman 24th. Smith didn’t quite reach that minimum workload before getting hurt, so drop it down to 10+ innings and he was 28th out of 296.

Even the two veterans at the end of the list are probably being underrated from this perspective. Petit has spent the last three years consistently beating his FIP in Oakland, by more than a run total, so you’re free to bet against him posting another 2.73 ERA at your own risk.

As for Romo, he always has gaudy K/BB rates but tends to give up a few extra homers, which hurts his FIP even more than it does his ERA, but moving to the Coliseum has a way of mitigating that particular weakness. At ages 36 and 38, one day the magic will run out for Petit and Romo, but it hasn’t happened yet.


2020 MLB reliever average: .318

  • Rosenthal: .216
  • Smith: .220
  • Diekman: .238
  • Kolarek: .254
  • Wendelken: .271
  • Trivino: .291
  • Romo: .312
  • Petit: .324

This time Petit just misses the league mark, but again, he’s been out there chucking 89 mph for years and it keeps working. He’s sort of the opposite of the Seattle Mariners, in that no matter what anything says on paper, I’ll believe he’ll stop being good when I see it. (One caveat, though, is that Petit’s xwOBA did rise last year by around 50 points from his long-term norm.)

On the other hand, the top of the list is even more distinguished. Rosenthal’s number ranked third among all relievers (minimum 40 batters faced), behind only Devin Williams and Drew Pomeranz. Smith was sixth, behind only Caleb Ferguson and Andrew Miller. Diekman was 19th, ahead of names like Hendriks, Zack Britton, Edwin Diaz, Raisel Iglesias, Brad Hand, and Aroldis Chapman.

Out of 306 relievers at this sample size, Kolarek ranked 36th, Wendelken 63rd, and Trivino 107th. That adds up to three relievers in the top 20 of xwOBA, and six in the top one-third percentile or so. And the other two are long-established veterans with some of most consistent track records in the sport, even when the predictive metrics haven’t liked them in the past.

No weak links

There simply aren’t any weak links in this bullpen entering the season. Of course, things can immediately go wrong, from someone(s) getting hurt, to an expected star falling apart for no apparent reason — whether permanently or just as a one-year blip. But in terms of laying the best possible plans in March, these are top-notch blueprints.

Even the arms who didn’t quite make the cut rank well in these stats. Right-hander Jordan Weems is waiting in the wings in Triple-A, after marks of 3.05 FIP and .303 xwOBA last year. And lefty Nik Turley got waived despite posting a 3.88 and .291 for the Pirates in 2020. Those are pretty close to Wendelken and Trivino numbers, respectively, and there just wasn’t room for them.

The 2021 A’s are effectively returning their MLB-best bullpen from last year, except with Hendriks Lite in the 9th inning and a slightly upgraded list of seven names behind him. If their strikeouts, walks, and batted balls are any indication, Oakland’s relievers have the chance to be excellent again.