Is there anyone having a weirder season than Masahiro Tanaka? Dude is in the top fifteen when it comes to strikeouts minus walks, and yet he’s got an ERA over five. He’s been better recently! And he’s done it by… refusing to throw fastballs. Despite this wrinkle, he’s still giving up nearly two home runs per game. We haven’t even gotten to the weird day/night splits, but they’re part of the story, too. Weird.

Here’s a strange coincidence. On May 22nd, I wrote that — since batters are increasingly hunting fastballs and swinging dead red on every pitch in the era where strikeouts aren’t necessarily penalized — it was time for a pitcher to consider throwing 80% breaking balls. On May 20th, Tanaka started throwing breaking balls more than half the time. He’s thrown more than 60% breaking balls in four games since. He’s getting close.

To some extent, the idea is simple. Balls put in play off of sliders, curves, and cutters aren’t hit as hard. And batters are looking for fastballs. So don’t give them what they want!

Results Per Pitch Type Pitch BACON ISOCON Launch Angle Exit Velocity 4-Seam 0.302 0.160 15.8 88.4 Sinkers 0.319 0.153 5.8 87.7 Cutters 0.296 0.165 10.8 85.1 Sliders 0.299 0.147 11.9 85.0 Curves 0.303 0.137 8.7 85.2 Changeups 0.283 0.164 8.2 84.7 SOURCE: Statcast BACON, ISOCON = batting average, isolated slugging on contact

Coming into this season, Tanaka’s best pitches when it came to slugging results in play were his vaunted splitter, his cutter, and his slider. So he lined up well to be our guinea pig. Throw less of the bad stuff and more of the good stuff and the homer problem will go away!

Obviously it didn’t. The pumped up curve and slider are not the problem. He’s given up five homers on the two pitches combined since he started featuring them harder, but the combined .143 isolated slugging percentage is below league average and the best he’s got other than his now show-me four-seamer. The cutter’s .192 ISO allowed in that time frame is not great, but not killer.

The sinker, though. He’s giving up a .373 isolated slugging percentage and seven homers in half the number of pitches as his slider since he stopped throwing sinkers as often. Think of it. He’s thrown as few as three sinkers in a game since, and it’s still getting lit up. His splitter has taken a turn for the worse, too. And it bears out in the movement — his splitter is slower than it’s ever been, and it’s dropping less than it was earlier this season.

Still.. ever since he dropped his arm slot to its lowest point this season, he’s getting the best sink he’s ever gotten on his pitches. Why is he giving up homers on these pitches?

Are his day/night splits related? For his career, Tanaka’s strikeout minus walk rate gets 1% worse during the day, but his isolated slugging percentage allowed goes up from .158 to .195. His results are 15% worse than league average during the day and 9% better than league average at night. This year, it’s even nuttier — he’s 32% better than league average at night, and 159% worse than league average during the day. He’s allowing a .467 ISO during the day.

Does his stuff move less during the day? Not when it comes to drop. His sinker and splitter have almost exactly the same drop in day and night games. Does he leave his sinkers up in day games? No, the difference is half an inch in aggregate. Looking through heat maps just makes you realize how few sinkers he’s throwing these days, and somehow they’re still getting hit hard.

Does the weather affect him? Maybe. Look at this stark difference in launch angle and exit velocity during day and night games this year.

Tanaka’s Terrible Days Launch Angle Exit Velo Day Games 17.4 90.1 Night Games 5.6 87.5 SOURCE: Statcast

Batters have been putting the ball in the air with authority in day games this year against Tanaka. Let’s return to movement because something has to be happening here. Horizontal movement informs ground ball rate on the sinker, and, lo and behold, Tanaka’s stuff is flatter in the day when it comes to horizontal movement.

Masahiro Tanaka’s Horizontal Movement Day vs Night Day Night Difference Curve 7.5 8.4 0.9 Cutter 4.5 -0.9 -5.4 Four-Seam -6.4 -8.5 -2.2 Splitter -12.5 -13.3 -0.8 Sinker -14.5 -16.6 -2.1 Slider 3.4 2.2 -1.2 SOURCE: Andrew Perpetua

Remember we were looking for reasons that his cutter, splitter, and sinker would give up more homers during the day. Looks like his sinker and cutter have wide differences in movement in the daylight vs at night. His cutter goes from having cut to… backing up and moving to the arm side.

They why is not obvious. Perhaps it has to do with grip on the ball, though usually warmer weather is more humid, which makes it easier to grip the ball. Perhaps it has something to do with the interaction of his new arm slot, new sink, and less sink during hotter starts.

Let’s return to the overall, unsplit results, then. There’s evidence that his newest mix — light on sinkers and heavy on splitters and sliders — is the best one for him. It’s the best combination of strikeouts and ground balls at least.

Tanaka’s Pitching Mix & Results by Period Sinker Cutter Slider Curve Launch Angle K% 4/2-5/14 27% 11% 22% 6% 8.1 17.7% 5/20-6/17 24% 6% 35% 6% 10.6 28.3% 6/23-7/28 14% 9% 33% 8% 6.8 26.4% SOURCE: Statcast

This seems easier to hold on to. Ever since Tanaka dropped his arm slot, his slider has had above-average whiffs and hasn’t been a problem on balls in play. That’s given him, with his cutter, three breaking balls that he can throw in lieu of his sinker, which is getting throttled. He’s come close to solving that problem by throwing the sinker less and the breaking balls more.

The last bit? The home runs? The day game problem? Turns out his stuff is straighter during the day. The why, though, that remains elusive. Told you it was a little weird.