Chris Sale has obviously been the pitching story in Boston this year, as the team’s first-year ace surpassed expectations and stabilized a group that saw David Price miss most of the year and Rick Porcello go backwards once again. But in Sale’s shadows, Drew Pomeranz has also been quietly excellent, and is one of the main reasons the Red Sox are almost certainly going to win the AL East this week.

Among AL starters, Pomeranz ranks 5th in ERA-, 11th in FIP-, and 10th in xFIP-. His season line is a near identical match for that of Justin Verlander, for example. For all the heat Dave Dombrowski took for giving up Anderson Espinoza to acquire Pomeranz last summer, he’s been more than worth the price thus far.

But, of course, one of the reasons people weren’t thrilled about the trade is that Pomeranz hasn’t proven to be a durable starter, and there are always concerns about his health in the postseason, when the Red Sox need him most. And once again, as the calendar turns to fall, the Red Sox can’t be sure whether they can trust Pomeranz with the ball next week.

The primary concern, in one pretty obvious picture.

Velocity bounces around from start to start, but those lines that look like elevators going straight down are always worrisome. And when Pomeranz loses a few ticks on his fastball, the results go south too.

Through August 23rd, Pomeranz had a 9% BB% and a 25% K%. Since then, however, he’s at 11% BB% and 17% K%, and in his last start, he failed to strike out any of the 13 batters he faced, as the Blue Jays lit him up for five runs in two innings of work.

As pitchers go, Pomeranz isn’t that complicated. He’s primarily a fastball/curveball guy, and he gets out by changing eye levels up and down. Here’s where he throws his fastballs.

And here’s his curveball.

The curveball is dependent on Pomeranz keeping guys honest with the fastball. Slow the fastball down a few ticks, and now hitters have all the time they need to adjust to the breaking pitch. And over the last four starts, hitters haven’t been fooled by the curveball.

Pomeranz’s Curveball Results Curveball Called Ball% Called Strike% Whiff% Through 8/23 34% 36% 11% Since 8/23 42% 22% 5% SOURCE: Brooks Baseball

And, likely not coincidentally, as Pomeranz’s velocity has gone south, he’s started mixing in his two-seam fastball more, de-emphasizing the high four-seamer against right-handers. Up through August 23rd, just 7% of his pitches to right-handed batters were sinkers, but in his four starts since, he’s at 19% sinkers. In fact, it’s his most common first-pitch to RHBs over the last month, as he’s starting righties with a sinker 30% of the time.

That’s a problem, because his sinker is kind of garbage. He’s only thrown it in the strike zone 39% of the time this year, and hitters almost never chase it out of the zone, and they make contact almost every time they do chase, since it doesn’t have swing-and-miss movement. Batters are hitting .389/.542/.639 against his sinker, and it’s been worth -6.2 runs relative to an average pitch despite having him only thrown 247 of them this year. Pomeranz has thrown one of the worst two-seamers in baseball this year, but lately, he’s upped its usage because the alternative is throwing elevated 89 mph four-seamers.

Right now, Drew Pomeranz basically doesn’t have a good fastball, which makes his curveball not work, which means he’s in trouble. And that’s a problem for the Red Sox.

With David Price working in relief since returning from the disabled list, the team’s rotation after Sale is a hodgepodge of guys with question marks. Eduardo Rodriguez has pitched his way into the #2 spot, in all likelihood, as September has been his best month of the year. But Porcello has also saved his worst for last, getting torched in September, and for all the good work Doug Fister has given the Red Sox this year, he’s still a soft-tossing righty, and managers generally don’t trust those guys much in the postseason.

If Price was stretched out and healthy enough to give the team five or six strong innings, this wouldn’t be much of a problem, as the team could tandem-start Pomeranz and Fister in Game 4, and rely on their other three starters and a strong bullpen to carry them through. But with Price in the bullpen, Pomeranz would have been one of those three starters get two starts per series. Unless he finds his fastball soon, though, I don’t know how the team can rely on him in that kind of critical role. If the choice is the current version of Pomeranz or whatever Fister can give you, you probably have to give Fister two starts in a seven game series.

Now, of course, it’s possible that Pomeranz’s fastball can come back. Maybe if they just give him some extra rest, he’s back to the guy he was in April through August, and the Red Sox will have a third quality starter to throw. A few bad weeks doesn’t mean he’s definitely screwed for the postseason.

But he probably needs to show some 92s and 93s with his four-seamer in his last start of the season. If he comes out again leaning on his lousy two-seamer, then Pomeranz might be a left-on-left reliever only in the postseason. And at that point, David Price probably needs to go back into the rotation.