Profile: Kohei Arihara

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Kohei Arihara

His injury-activation report is posted on Inside Pitch, but there’s not much there to see.

Team: Nippon Ham Fighters

Current status: On Wednesday, Dec. 4, 2019, Arihara told a press conference after signing his 2020 contract that he informed the Fighters he wants to be posted.

Pos: SP Birthdate: Aug. 11, 1992 Throws: R

NPB page

Honors: 2015 Pacific League rookie of the year

League leader: Wins: 2019

Arihara is smack in the middle of a continuum that stretches from Yu Darvish, who throws EVERYTHING and has a real good fastball, and Masahiro Tanaka, who CAN throw everything but who will tend to focus on two or three at a time in his quest for the perfect combinations.


Arihara’s breakthrough 2019 season was largely a result of a) being able to confound batters with lots of different offerings, and b) finding optimal combinations. His three secondary pitches, slider, change and split all being as good or better than ever, and a big change was his decision to use them more. He more or less ditched his cutter and threw his fastball half as often.

Arihara had a spectacular first half and finished runner-up in the PL’s Best Nine voting at pitcher behind Kodai Senga of the Hawks. Through June he struck out 91 batters in 90 innings while walking 17. From July he struck out 70 over 74-1/3 innings while walking 23.

The Defensive Efficiency Ratio behind him in 2019 was .752, vastly higher than his norm of around .680, but so was the percentage of balls Delta Graphs categorized as hard contact we went from hovering around 35 percent to 29 percent.

My homework

Here is an analysis of Arihara’s 2019 first half I did for my Sports Management World Wide analytics course:

Arihara was the Pacific League’s 2015 rookie of the year when he went 8-6 largely because of tremendous run support (He had a 4.79 ERA in a pitchers park!) and said famously at his award speech that he would endeavor to be worthy of an award he didn’t deserve to win.

Through 11 games this season, he’s 8-2 with an ERA almost two runs lower than his career 3.91 with a huge increase in strikeouts, that some have attributed to his working with one of Japan’s leading changeup practitioners, Chihiro Kaneko.

Pitch selection

Arihara had been throwing four-seam fastballs over 40 percent of the time, this year it’s been down to 30 percent with most of those pitches being replaced by two-seamers, something of a novelty pitch in Japan. He throws a slider (14%), a cutter (13%) and has increased his changeups to about 18 percent of his total while cutting down on his splitters (10%).

Pitch value

Whether the impact has been from Kaneko joining the Fighters or not, Arihara’s results with his changeup are now rated by Delta Graphs as the best in NPB, while his fastball has gone from a necessary evil to a bit better than average and his slider from mediocre to better than average.

Plate discipline

Arihara does not appear to have changed his approach at all, although his pitches are generating fewer swings (47.3 percent) – the lowest figure since his 46.9 percent as a rookie.

The huge difference is that he is MUCH harder to make contact against this year. Prior to this season, he was missing bats on about 9 percent of his pitches. This year it’s up to 14 percent while getting fewer foul strikes than before. Last year, batters made contact on 80.6 percent of Arihara’s pitches, the lowest figure of his career, until this season’s 70.5 percent.

What’s different for 2020

Arihara had a very difficult 2020 season, and while I’m not looking at the results for each pitch, it sure seems that the big difference this year is his inability to get guys to chase and miss. Last year, he ranked second in lowest contact percentage outside the zone. This year, he’s fourth highest among ERA-title qualified pitchers.

Arihara still gets swings and misses, but he is now only better than average among Japan’s top pitchers.

Arsenal 2020

PitchUsage %Avg. VelocityValue per 100
Cutter 13.8 140.5 +0.25
Two-seam 12.2 145.1 +0.03
Slider 11.4131.1-2.40

Arsenal 2019

PitchUsage %Avg. VelocityValue per 100

writing & research on Japanese baseball