The most frustrating part about Masahiro Tanaka's decline from having the third-best ERA in the American League (3.07) to the second-worst mark in all of baseball (6.55) isn't just that the pitcher the Yankees considered to be their ace seems to have disappeared. It's that this is all happening without any clear reason why, and as we attempt to figure out what's going on, we should warn you in advance that this won't be satisfying.
Let's not kid ourselves here; you're thinking about Tanaka's arm. It's well-known that he's pitching through a partial tear in his right elbow, and while that didn't seem to bother him in 2016, a fall this stunning must make you immediately wonder if the issue is now preventing him from succeeding. But both player and team insist there's no injury, and there's no noticeable decline in velocity, though Hall of Famer Pedro Martinez (who knows a thing or two about pitching) voiced his concern even before Tanaka allowed three homers to the Red Sox in Tuesday's 5-4 loss.

But is Martinez really right? This is where it gets complicated. At his best, Tanaka gets hitters to flail at sliders and splitters out of the zone, and since he is clearly not at his best or anything like it right now, one might assume that's not the case, that hitters aren't chasing and are instead sitting on his solid-but-not-elite four-seam fastball.
But the numbers tell a different story, which is frustrating. In 2014, when Tanaka arrived in America and took the Majors by storm, he led all pitchers who threw at least 100 innings with a 37.6 percent chase rate. Last year, when he was very good, it was an identical 37.6 percent. This year, as Tanaka is getting crushed, it's a very similar 36.8 percent -- the fifth highest. Not only that, hitters are making less contact on outside-zone pitches -- only 60 percent as compared to last year's 64 percent.
In theory, that ought to be a good thing. You always want a hitter to expand his zone, and inducing more swings and misses on those non-strikes generally is a happy outcome for a pitcher. Of 85 starting pitchers to throw at least 250 pitches, Tanaka's .314 outside-zone wOBA* allowed is 34th, and while it's definitely a problem that it's higher than last year's .230, it's not the problem.

(*wOBA is Weighted On-Base Average, which is very similar to on-base percentage except it gives increasingly more credit to extra-base hits as opposed to treating every time on base equally, as OBP does. The 2017 Major League average wOBA is .318.)
No, the problem is what happens when Tanaka's in the strike zone. Of those same 85 starting pitchers, only one has a higher in-zone wOBA allowed than Tanaka does, and he's surrounded on both sides by soft-tossers past their 40th birthdays.
Highest In-Zone wOBA allowed by a starting pitcher in 2017
.434 -- Bronson Arroyo, Reds
.423 -- Masahiro Tanaka, Yankees
.422 -- Bartolo Colon, Braves
.410 -- Patrick Corbin, D-Backs
.401 -- Ricky Nolasco, Angels / Mike Fiers, Astros
MLB SP average -- .339
It's the second-largest in-zone wOBA jump of any starting pitcher. Of course, all this is doing is reinforcing your belief that Tanaka has been getting shelled, but it doesn't answer the important question of why that is. And while there are plenty of little reasons, the largest seems to be, very simply, inconsistency.
As we said above, that's unsatisfying, but then again,Tanaka's season has been, too. If he was simply hurt, you'd see it in the velocity. You don't. Or you'd see it in the spin rate. You don't. You'd see a pitcher who was getting crushed each time out, but Tanaka outdueled Chris Sale to throw a three-hit shutout in Fenway Park back in April, and he generated a career-high 25 swing-and-misses while whiffing 13 A's barely more than two weeks ago. He's obviously still got the ability to dominate good hitters. Sometimes.
It's how in the same game, Tanaka could perfectly spot a pitch over the outside corner for a called strike three on Pablo Sandoval ...

...and then leave a cement-mixer slider spinning right over the plate with no movement for Mitch Moreland to crush into the seats.

It's how Tanaka's swinging-strike rate could actually be up from last year, from 10.9 percent to 12.7 percent -- 10th best in baseball, better than Stephen Strasburg or Yu Darvish -- yet find his overall strikeout rate down, from 20.5 percent to 19.7 percent, slightly below average for a starter. The only way that happens is if you still have the ability to induce misses, but not the consistency to string enough together to actually finish the hitter off.
It's that word, inconsistency, that keeps coming up. Manager Joe Girardi used it after the game, saying "for him, it's been inconsistent stuff that's really killed him." Tanaka used the word, too, but it was something else he said that really resonated.
"As for today, I think I made some good pitches and bad pitches in the game," Tanaka said. "The bad ones, I think I missed. They were crucial mistakes, and they went right down the middle."
That's exactly right. Not all of Tanaka's pitches have been bad, but there's been enough of them to kill him. It's exactly what pitching coach Larry Rothschild has been working in doing things like having Tanaka shift where he stands on the rubber. As Rothschild said late last month, "fastball location is important, because when he misses with it, it's just not a good pitch."
Tanaka has never been the type to blow hitters away. He's relied on movement and location, and right now, he's been unable to find consistency on either. You understand why Yankees fans are growing impatient and would rather see Luis Cessa, Chad Green or Chance Adams take Tanaka's next start. Perhaps a break, either mental or physical, is in Tanaka's best interest.
Yet as frustrating as this all is, you understand why the Yankees still see hope. In his good games, he's the Tanaka of old. In his bad games, he still shows flashes of what was. It's why claims that there's not an injury seem to be valid. It's why the Yanks will do whatever they can to get Tanaka back on track. Though it's hard to see now, the talent is still there.