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What makes Masahiro Tanaka better on extra rest?

A major storyline in 2016 has been Masahiro Tanaka's significantly better numbers with extra rest, but what specifically is causing the disparity?

Brad Penner-USA TODAY Sports

Joe Girardi is getting tired of hearing about Masahiro Tanaka's improved performance on extra rest, a recurring storyline throughout the 2016 season. In an interview a couple of weeks ago, Girardi was asked about Tanaka's rest-based splits and essentially responded with the shrug face emoji, according to NJ.com.

With the occasional rumor of a six-man rotation, the feeling is that if it were feasible, the Yankees would have done it already. Still, the inevitability of short rest outings for Tanaka is worrisome. According to Baseball Reference, the Yankees' ace has a 5.33 ERA on four days of rest, and a 1.70 ERA on five or more days in 2016.

So why does Masahiro Tanaka do so much better with an extra day of rest? The easy answer would be to point to the partially torn UCL in his throwing arm. Tanaka has said in interviews that he feels fine, and is backing it up on the field. Hopefully, we are past the "should've gotten Tommy John surgery" narrative. Also, he actually pitched better on less rest in 2015, with a bone spur in his right elbow.

Mechanically, Tanaka appears to be remarkably consistent. Looking at pitch-by-pitch Statcast and Pitch F/X data, his velocity, release points, spin rate, and extension are all pretty similar regardless of rest. The only noticeable difference is his differing pitch selection:

Pitch Type

4 Days Rest %

5 Days Rest %

6 Days Rest %

CU

5.24%

4.67%

5.54%

FC

10.22%

7.88%

6.93%

FF

3.28%

1.31%

5.26%

FS

30.67%

29.64%

36.57%

SI

21.63%

25.84%

24.93%

SL

28.96%

30.66%

20.78%

With more rest, he tends to use his splitter a bit more, opting for his slider more often with less rest. The difference is still pretty small though, so it could be more of a function of facing more predominantly right-handed lineups on less rest. It could also be because his splitter command is worse on short rest, looking at FanGraphs' Pitch Values metric. On four days rest, his splitter has been worth 0.9 runs above average, compared to 8.2 RAA on five or more days. To be fair though, his slider and sinker also have worse pitch value scores on four days of rest.

His peripheral stats also seem to point to the idea that he is just somehow better with extra rest:

Rest

IP

FIP

K/9

BB/9

HR/9

LOB%

WHIP

4 Days

49.0

4.40

6.98

1.84

1.47

58.8%

1.388

5+ Days

74.0

2.57

7.54

1.58

0.36

84.4%

0.892

After getting burned by home runs for most of last year, he has actually done a great job of keeping the ball in the yard with extra rest. He also is much better at stranding runners, which could suggest issues out of the stretch on short rest. His significantly lower WHIP suggests that he has also been better at keeping runners off the basepaths in the first place.

Looking at the numbers, it is difficult to explain how Tanaka is better with extra rest without saying "He just is." His similar mechanics and strikeout and walk numbers suggest that things could even out over a larger sample size. Things like his left-on-base percentage could smooth over after more starts, especially considering his highly touted competitive streak. Keep in mind, Tanaka was actually better with less rest in 2015. For now, the Yankees will have to hope their ace can find a way to be more consistent.

*Data is courtesy of FanGraphs, Baseball Savant, and Baseball Reference.