When the Mariners signed Japanese lefty Yusei Kikuchi before the 2019 season in a deal that has the potential to max out at seven years and $109 million, they were hoping to acquire a top of the rotation starter. In the previous year with the Seibu Lions, he pitched to a pristine 3.04 ERA while averaging over eight strikeouts per nine innings. It was a move to give a retooling team some stability in their transition.
But things went awry in his debut season, as by nearly any metric, he was far below the league average. His 5.71 FIP produced value just a shade above replacement level, and his 5.18 xFIP didn’t paint a very good picture of what the future might hold. It simply appeared that Kikuchi’s success in the NPB may not translate to Major League Baseball. Fortunately for both he and the Mariners, things took a drastic turn for the better in 2020.
While his ERA only decreased modestly from 5.46 in ‘19 to 5.17 in ‘20, Kikuchi improved in many important metrics in his 47.0 inning sample. For example, his K% skyrocketed from 16.1% to 24%, lowering is FIP to a near elite 3.30 (75 FIP-). Now, It would be both natural and perfectly reasonable to take these improvements with a grain of salt, considering the nature of this past season.
To which I would respond, not so fast...
Kikuchi made vast changes to his pitch arsenal, and they proved immediate dividends. For one, he ditched his curveball and added a low 90’s cutter, trading a pitch with a .431 xwOBA and a 15.4% Whiff% for a much better one with a .288 xwOBA and a 25.0% Whiff%. He had faith in the cutter right away, relying on it 40% of the time.
His fourseam fastball also improved twofold. Kikuchi not only increased his fastball velocity from 92.5 mph to a well above league average 95.0 mph, he also increased its active spin (the percentage of spin that contributes to a pitch’s movement) from 78% from 92.4%. This effectively added four inches of rise, making the pitch elite for its class. As a result, his xwOBA on the pitch dropped from .381 to .308 and his Whiff% increased from 16.0% to a eye popping 31.0%.
Because of these two new and improved pitches, Kikuchi relied on his slider much less in ‘20 than he did in ‘19, but even it was better than the previous year’s offering. Adding seven inches of drop, the pitch generated a nearly 40% Whiff% and a pedestrian .200 xWOBA.
In one year (albeit a strange one), Yusei Kikuchi went from well below average to well above average in most important measurable categories, even rating near elite in a few. The 2021 season will be an important season for him, as the end of this year presents a fork in the road for his contract, where he can either exercise a one year player option, or the Mariners can choose to give him a four year extension to push the pact to its maximum value.
If Kikuchi can carry his ‘20 successes into ‘21, it should be a no brainer for this young and talented Mariner organization to hang on to its star pitcher.
Brian Menéndez is a contributing writer for Beyond the Box Score, as well as a senior writer for DRaysBay. Additionally, he has been featured in The Hardball Times. You can find Brian on Twitter at @briantalksbsb.