As we creep up on the beginning of May, the Milwaukee Brewers have the best record in baseball. At 19-7, they’ve thrust themselves squarely into playoff contention, even if the pre-season projections mostly saw them as a third wheel in a difficult division; their early success combined with the struggles of their direct competition have opened the door for the Brewers to make a real run at the postseason. As Jeff noted two weeks ago, it doesn’t even matter all that much that our projections still aren’t that bullish on their future performance, because the cushion they’ve created with a strong first month of the season gives them plenty of room to regress and still be in contention.

Which is a good thing, because there’s almost certainly some pretty harsh regression coming the Brewers direction; one of the core foundations of their strong start has been a remarkable performance from their bullpen.

The red line is the Brewers total Win Probability Added by their relievers during the first 26 games of the season. The Padres are a close second, but then there is an enormous drop-off after those two, with the third highest WPA from a bullpen unit (SFG) checking in at +2.46, while the fourth highest (BOS) is just +1.76, less than half of the Brewers’ +3.98 total bullpen WPA to date.

Let’s put that number in some context. A couple of years ago, the Orioles made an improbable playoff run based almost entirely on the remarkable performance of their relievers, who posted the highest single season team bullpen WPA in the 40 years of history that the metric covers; their +13.86 reliever WPA was actually more than +2 WPA better than the previous record holder, in fact. The 2012 Orioles bullpen was worth about an 8.5% win probability boost per game over the course of a full season.

The Brewers’ have amassed +3.98 bullpen WPA in 26 games, which works out to a 15.3% win probability boost each game; that’s 80% more impactful than the most impactful bullpen of all time. At their current pace, the Brewers would amass almost +25 WPA from their relievers; the 2012 Orioles were the first team to ever get past +12, and only 12 teams (out of 1,100) have managed to break +10.

Francisco Rodriguez is getting all the headlines, as he looks like vintage K-Rod racking up saves left and right, but this has really been a team effort. Beyond Rodriguez — who leads the majors in reliever WPA — the Brewers have two other pitchers in the top 10: lefties Zach Duke and Will Smith. Yes, that Zach Duke, the one who has spent his career as a pitch-to-contact swingman, has spent the first month of the season as a dominating setup guy. Somehow, at age-31, he’s decided to start blowing hitters away, and has posted a 34% strikeout rate over the first month of the season, and he’s not just being used against lefties either.

Between them, Dukes and Smith have thrown 33.2 innings and allowed a grand total of two runs, and neither of them had any effect on an outcome of the game. Dukes gave up both of the runs, allowing the Braves to increase their lead to 4-2 in Game 2 of the year, and then giving up a run to cut their lead over the Phillies to 6-4 a week later. They wouldn’t score again against the Braves, so the run proved meaningless, while his teammates shut down the Phillies for the next three innings and added four more insurance runs in an eventual blowout.

And it doesn’t just stop with the left-handers. Tyler Thornburg has been the primary right-handed setup guy for the team, and he’s allowed one run all season; the final (also inconsequential) run on that 5-2 Game 2 loss to the Braves. The Brewers four most used relievers this year have combined to allow a grand total of three runs, and all three came in games where the run proved to have no impact on the outcome. Four pitchers, 51.1 innings pitched, and no meaningful runs allowed. This is how you rack up +3.98 bullpen WPA in 26 games.

We can talk about Charlie Blackmon being on pace for a +12 WAR season as the most obvious sign that early season performances shouldn’t be extrapolated, but there is no performance anywhere in baseball that is further from expectation than the Brewers bullpen. Keep in mind that last year, the Brewers finished 28th in bullpen WPA, and their unstable relievers were one of the main reasons the team underachieved. In 2012, the Brewers were 27th in bullpen WPA. This has been the team’s achilles heel for a couple of years now, and now, it’s the area that is almost single handedly carrying them to the best record in baseball.

That isn’t meant to discount the rest of the talent on the team. Jonathan Lucroy may very well be a stealthy MVP candidate, given his combination of offense and impact behind the plate, and with Carlos Gomez, Ryan Braun, and Aramis Ramirez anchoring the offense, the guys are going to score some runs. Matt Garza was a solid off-season addition, and while the rotation lacks any kind of true ace, it also doesn’t have any batting practice pitching machines. This team is a lot better than last year’s stars-and-scrubs failure.

But this Brewers team is being carried by its bullpen, and more specifically, the bullpen’s total shutdown of opponents in important situations. Team wOBA allowed, by leverage situation:

Low leverage: .295 (10th)

Medium leverage: .271 (3rd)

High leverage: .198 (1st)

wOBA allowed in high leverage situations will naturally be lower, because that’s when the good relievers are put in the game, but this is not a sustainable performance. The Pirates posted the lowest team wOBA allowed in high leverage situations last year, and they came in at .243; the Braves were second lowest at .265. Even if Francisco Rodriguez really is the best closer in baseball, and Zach Duke has been reborn as a dominating one-inning bullpen weapon, and Smith and Thornburg are going to continue to pitch at a level near the game’s best left/right setup tandem, the Brewers bullpen would still be in for a huge performance regression, simply because of the distribution of when they’ve allowed opponents to get hits.

Once you factor in the reality that K-Rod isn’t going to keep pitching like Craig Kimbrel, and Zach Duke is probably still Zach Duke, and well, the May-September Brewers are going to have to find a new way to win baseball games. Maybe that means Jean Segura starts hitting again, or they find a real first baseman somewhere, or Khris Davis remembers that he’s supposed to get on base occasionally. There are areas for the Brewers to get better, so we shouldn’t just assume that everything except their bullpen is going to hold steady while that group regresses to the mean.

But the impact of the Brewers bullpen on their first 26 games can’t be understated, and Khris Davis would have to turn into Babe Ruth to offset the coming reliever regression. The Brewers current recipe for winning isn’t going to continue to work at the rate it has so far, and they’re going to have to find a new way to beat teams that doesn’t involve protecting every lead they ever get. The good news is that the wins they’ve already accumulated aren’t going to get revoked, and even if they play .500 ball the rest of the way, their April performance will keep them hanging around all year long.

But if you’re buying the Brewers as an elite team based on their April performance, just be aware that you’re betting on the sustainability of a team posting the greatest bullpen performance of all time, and getting there by approximately the All-Star break. I think I’ll take the under on that happening, and we’ll have to see how well the Brewers can play when their relievers remember than they’re human.