If Masahiro Tanaka was a protagonist in a Greek tragedy, his fatal flaw would be allowing home runs. The Wild Card game against the Astros was actually a very typical Tanaka start, as he hit his spots for almost all of the game. However, he allowed two solo shots, which was more than enough for Dallas Keuchel and the Astros to advance to the ALDS.
At the heart of Tanaka's home run problem were the video game numbers that batters put up against his fastball and sinker. According to Brooks Baseball, hitters had batting averages over .300 and slugging percentages north of .600 against both pitches. In total, he has allowed 17 home runs on his two seamer and four seamer. Fortunately, Tanaka managed to salvage the season by making his fastball and sinker the third and fourth most common pitches in his arsenal, behind his famed splitter and under the radar slider.
Throwing fastballs just 32% of the time might not be a sustainable model for a starting pitcher though, especially for those expected to anchor the rotation and pitch deep into games. Fellow Japanese pitcher Hiroki Kuroda managed to carve out a very underrated career with the Yankees relying primarily on his splitter, but he did not have a partially torn UCL in his throwing elbow. If Tanaka wants to maximize his potential as a pitcher, he will need to find a way to use his fastball and sinker effectively.
In 2014, New York Times writer Tyler Kepner wrote a piece comparing Tanaka to Hall of Fame pitcher Tom Seaver. Kepner called Tom Seaver the "godfather of the drop-and-drive delivery," a pitching motion that is easily identifiable by a long stride towards the plate. Drop and drive pitchers will allow their bodies to fall towards home plate after they lift their front leg, generating momentum, and in turn, velocity. By releasing the ball closer to home plate, the perceived velocity on their pitches is greater.
Interestingly, Tanaka somehow doesn't release the ball closer to the plate. Statcast pegs Tanaka's average extension on his splitter at under six feet, which is shorter than his height of 6'2". At first, that doesn't seem possible, since his stride length is estimated to be 1.1x his height in the New York Times article. Statcast's extension figure is actually defined as "the ball's distance from the rubber, in feet, upon the pitcher's release," according to a post at MLB.com. In other words, Tanaka's release point is further away from home plate than other pitchers who have the same stride.
The disparity between his stride length and short extension can explain why his fastball and sinker are hit so hard. Major League pitcher's mounds are supposed to be ten inches above the ground, giving pitchers a natural "downward plane" on their pitches. Scouts will often refer to the downward plane when talking about tall pitchers, saying it helps them to keep the ball on the ground.
Pitchers with drop and drive deliveries essentially give up some of that downward plane. However, they are usually trading it for a greater perceived velocity. While a 93 mph fastball could look like 95 mph coming from a drop and drive pitcher, assuming he uses his long stride to release the ball closer to the plate, Tanaka's fastballs end up looking slower than they actually are. Combined with the significantly straighter plane created by his long stride, Tanaka's low 90's fastballs are extremely inviting for hitters.
Because of this anomaly, Tanaka fails to do what he is supposed to with his fastball and sinker. Since 2014, hitters have only whiffed at just 5.4% of his four seam fastballs and have a relatively low groundball rate of 45% against his sinker, according to Brooks Baseball.
In order to get out of this mechanical no man's land, Tanaka will probably have to either change to a "tall and fall" delivery or move his release point forward. Unfortunately, it is likely much, much easier said than done. Tanaka is 27 years old, and his world-class command is probably built on repeating the mechanics he has had for the majority of his career. There is also no telling how much of his deadly splitter's success is predicated upon his mechanics. Because of his importance to the Yankees' rotation, he can't just go experimenting with new mechanics. But if he finds himself struggling in the future, it Tanaka might have to change things up.