Yusei Kikuchi Could Make the Mariners’ Rotation Compelling

The Mariners’ starting pitchers can be broken into two different groups. There are the veterans at the top in Marco Gonzales and James Paxton, who between them have spent about 10 seasons in Seattle. They’ve both been successful, and so long as they stay healthy — not a given for Paxton — we know what to expect from them. The other half of the Mariners’ rotation is the young guys: Justus Sheffield, Justin Dunn, Logan Gilbert, and others. Each of them had or has considerable prospect stock, but while they’re expected to be successful at some point in the future, we can’t be sure that success will arrive in 2021.

Holding those two groups together in the middle is left-hander Yusei Kikuchi, fittingly, as he represents something from both. At 29 years old and with nine seasons spent in NPB, he’s already a veteran. But as with the young guys, there is still a lot to learn about how he will fare in the majors, given that his MLB experience amounts to two years. The first was 2019, his rookie season and first time pitching in the U.S., when he posted a 5.46 ERA and 5.71 FIP in 161.2 innings as Seattle finished last in the AL West.

The pandemic-shortened 2020 season, though, suggested there could be a lot more to him. Kikuchi’s ERA stayed above 5.00, but his FIP plummeted to 3.30, and he made significant progress with his strikeout and home run rates. If his surface numbers take the step forward this year that his peripherals did last year, it would be a major boost to the Mariners as they try to crawl toward contention.

If you want to believe in Kikuchi as a breakout candidate for 2021, there is no shortage of data to back you up. The improvement in his numbers last year were accompanied by a substantial increase in his fastball velocity, from an average of 92.5 mph to 95. Additionally, the perceived rise of his four-seamer went from almost exactly average up to the 90th percentile. See if you can tell the difference:

2019

2020

The velocity increase showed up when Kikuchi reported for spring training last February. Per the Seattle Times‘ Ryan Divish, the lefty spent his first MLB offseason at Driveline shortening up his arm action to make his mechanics work more smoothly. The impact on his fastball was dramatic: Its whiff rate nearly doubled, from 15.9% to 30.8, and opponents’ expected slugging percentage against it fell from .525 to .354. That’s a lot of progress to make in a single year to shore up a major weakness, and what’s perhaps even more impressive is that the four-seamer was still arguably his worst pitch. It ran a .330 wOBA and .319 xwOBA, both of which were the highest among his offerings.

This all looks completely different from 2019. The changeup was hit harder in his first season, his slider didn’t miss nearly as many bats, and the cutter didn’t exist at all. When Kikuchi first arrived in the majors, his four-seamer was his only fastball, and his favorite go-to secondary pitch wasn’t a slider but a curveball. But over the course of the season, he methodically phased the curve out, becoming primarily a four-seam/slider pitcher by August, similar to how many left-handed relievers operate.

When the 2020 season began, the curve was nowhere to be found, and a brand new pitch was suddenly his favorite offering. He used the cutter on 42.9% of his pitches in his first start of the season and continued to lean on it hard throughout the year. It’s another pitch that relies more on velocity than movement; its 92.1 mph average is about nine miles per hour better than his slider. Here’s what the two pitches look like next to each other:

With his new arsenal, Kikuchi raised his strikeout percentage from 16.1% to 24.2, slashed his homer rate from two per nine to 0.57, and raised his ground-ball rate from 44% to 52. And yet his ERA remained as mediocre as before, some 20% worse than league average. Fortunately, according to Divish, the Mariners seem to have an idea of how to get more out of him. It starts with improving the command he has over his revamped stuff. While batters didn’t make much contact against him last year, they did find success by waiting him out when he struggled to throw strikes. That was often: Kikuchi ranked in the bottom fifth among 2020 starters in percentage of pitches thrown while behind in the count. He also told Divish that he’s trying to get a better grasp on the mental side of his work after last year, and who among us is not?

If this is the year it all comes together for Kikuchi, it would be excellent timing. The contract he signed with Seattle before 2019 is an odd one. The first three years and $43 million are straightforward, but after that, the Mariners must either exercise a team option for four years and $66 million all at once or offer him a $13-million player option for 2022. The 2021 season is the all-important third year of the deal, when Kikuchi must prove he is worth an even bigger commitment than the first one Seattle made.

A Kikuchi breakout could also give the Mariners enough rotation depth to flirt with a wild-card spot (at least for longer than most people probably expect). You may not have noticed, but Seattle finished ninth in the majors in rotation WAR in 2020, 17 spots higher than the previous season. Surviving 162 games at a similar level in 2021 will be more difficult, but I don’t think it would be a major shock. We know how effective Gonzales and Paxton can be, and youngsters like Sheffield and Nick Margevicius pitched well in short samples last year. No one is going to mistake Seattle for San Diego, but the starters could be quite good for a team that otherwise seems far from postseason contention.

That will require a few of the unproven arms to hit the ground running, though, and the one who has the best chance of doing so is Kikuchi. Not only is he throwing substantially harder than he was when he got to MLB, but he’s also throwing with an arsenal that he seems much more confident in than what he arrived with stateside. He may stop looking like the teammates who are still getting used to the majors and more like an established veteran very soon.