On Monday, the White Sox lost 1-0, so that makes things fairly uncomplicated — they lost because they didn’t score. It happens. Also, they ran into Matt Harvey, and the Mets had reportedly identified a problem with Harvey’s mechanics beforehand, so if Harvey’s back on track now, well, there’s no shame in losing to him. Matt Harvey is an ace, and sometimes aces shut people out.

That’s how the White Sox lost their most recent game. Now let’s talk about the previous games.

Sunday, the White Sox played the Royals, and the White Sox led the Royals. Here’s the win expectancy chart:

When the White Sox bullpen took over, they had a win expectancy of 84%. Saturday, the White Sox played the Royals, and the White Sox led the Royals. Here’s the win expectancy chart:

When the White Sox bullpen took over, they had a win expectancy of 97%. Friday, the White Sox played the Royals, and the White Sox led the Royals. Here’s the win expectancy chart:

When the White Sox bullpen took over, they had a win expectancy of 86%. If you followed the series, I’m not showing you anything you didn’t already realize — there’s nothing subtle about a late-inning comeback, much less a few of them. The Royals have grown accustomed to such clutch-ness. As for the White Sox, it was more or less rock bottom for a bullpen unit that has struggled in May after a phenomenal opening month.

To an extent, you could say the White Sox have gone as the bullpen has gone. I don’t spend a lot of time focusing on Win Probability Added, or WPA, but I do like using it for relievers, since so much of their value is about context. And, the White Sox started hot. Through the end of April, they were 17-8, and they had baseball’s second-highest bullpen WPA. It’s not like that was the only thing working; the team had, and has, quality starters. But the bullpen hardly gave opponents anything. It had a WPA nearly two wins better than the average.

May has turned into a very different story. The White Sox’s run production is actually up, and the rotation has only somewhat regressed. Yet the team has a monthly record of 10-17, owing to the fact that the bullpen has baseball’s lowest WPA since the calendar turned. It’s not simply that a strength turned into less of a strength. It’s that a strength turned into a crippling weakness, with the May bullpen WPA being nearly four wins worse than the average.

Here’s a plot for every team, showing April bullpen WPA and May bullpen WPA. The relationship isn’t that tight, because bullpens function semi-randomly, but I’ve highlighted the White Sox because they still stand out.

Second-best, to first-worst. The White Sox aren’t totally alone; Oakland’s bullpen WPA dropped 3.2 wins between months. But the White Sox dropped 5.4 wins between months. The Astros improved by 3.1 wins, and the Brewers improved by 2.4 wins, and the White Sox got worse by about that same amount combined. No other bullpen showed that kind of WPA drop between months. No team’s rotation showed that kind of WPA drop between months. No team’s lineup showed that kind of WPA drop between months. The bullpen has been extremely volatile, and team records are sensitive to that, due to how bullpens pitch with higher leverage.

Of every White Sox reliever to pitch in April, only Jake Petricka and Zach Duke had a negative WPA. Of every White Sox reliever to pitch in May, only Dan Jennings has had a positive WPA. Matt Purke is in second, at an even 0.00. I don’t want to be too mean to anyone in particular when an entire unit has failed, but Matt Albers finished May with a WPA of -1.50, which is tied for the lowest mark in baseball for the month if I’m reading this right. We have these statistics called Shutdowns and Meltdowns. Click that if you don’t know them! In April, Albers had eight Shutdowns and zero Meltdowns. In May, he’s had one Shutdown and six Meltdowns. They’ve pretty much all been worse, so Albers hasn’t been the only villain, but he’s been the most villainous, and the White Sox have lost because of it.

When a bullpen fails, everything looks worse. Everyone’s more dejected, and that dejection colors perspectives, and fans start asking about the manager. It’s arguably the worst way to lose, but if it’s any consolation, we actually project the White Sox bullpen to be one of the better ones. And it was just good a month ago, so some of these lousy results can simply be dismissed as unfortunate. Nevertheless, the losses all count, and as Rick Hahn gets looking at rotation candidates like James Shields, he might also consider giving his bullpen a shot in the arm. The bullpen helped get the White Sox off to an outstanding start, but that raised expectations, and for a few weeks they haven’t been met. This can be salvaged, but it’ll take work in a competitive division. I know these are virtually empty statements but there’s an opportunity here, and Hahn can’t focus on the rotation alone.

Why did the White Sox suddenly start losing? Mostly because the bullpen gave out. That was made most evident over the weekend, and no team’s bullpen is really this bad, at least outside of Cincinnati. As the bullpen regains form, so, too, should the White Sox as a whole. But this does point to an area of potential improvement. There are a few places for the White Sox to add to their depth.