Chris Flexen in Korea: What changed?

On December 7, 2019, one day after he was designated for assignment by the Mets, Chris Flexen signed a one-year, $1 million contract with the Doosan Bears of the KBO, the maximum allowable sum a team is allowed to signed a rookie foreign free agent for. Having struggled in limited innings in 2019- and indeed, his entire professional career at the major league level- the monetary value may have come as a surprise for some.

In the end, the Bears scouting and analytics departments would prove to be prescient, as the right-hander would go on to post a 3.01 in 116.2 innings for the Bears, allowing 97 hits allowed, walking 30 walks, and striking out 132. While those numbers are well above-average— Flexen’s ERA was sixth in the league among qualified starters and his strikeouts tenth— there were periods of time where he was utterly dominant and otherworldly. In the month of October, Flexen allowed three earned runs over the course of five starts, posting a 0.85 ERA 31.2 innings with 19 hits allowed, 3 walks, and 42 strikeouts. In his start in the KBO playoff quarterfinals against the LG Twins, he threw six scoreless innings, giving up 4 hits, walking 1, and striking out 11. In the KBO playoff semifinals against the KT Wiz, he allowed two earned runs over 10.1 innings, allowing 5 hits, walking two, striking out 13 and notching a three-inning save. In the Korea Series against the NC Dinos, he allowed four earned runs over 12.0 innings, allowing 10 hits, walking 4, and striking out 8. All in all, he posted a 1.91 ERA in 28.1 postseason innings, allowing 19 hits, walking 7, and striking out 32.

So, what changed?

Admittedly, perhaps the biggest factor in Flexen’s success was something that he has no control over whatsoever. The KBO as a whole is inferior to Major League Baseball in terms of hitter and pitcher quality, generally considered to be somewhere between Double-A and Triple-A in terms of talent and competition level. Coincidentally, Flexen experienced major prospect helium in 2017, when he spent the first half of the season with the Binghamton Rumble Ponies, the Mets’ Double-A affiliate. In 48.2 innings, he posted a 1.66 ERA, allowing 28 hits, walking 7, and striking out 50, jumping from 21 on the 2017 Mets Top Prospects list to 4 on the 2018 Mets Top Prospects list.

In the Major Leagues, Flexen’s fastball averaged 93.3 MPH, making his pitch average to above-average in terms of velocity. In Korea, where the average fastball velocity sat at 87.7 MPH over the course of the 2020 season, Flexen’s fastball would be in elite territory, matched only by a handful of pitchers. Interestingly, his fastball velocity fell slightly, averaging 92.6 MPH, but even still, the speed of the pitch still firmly placed him among the top percentile of pitchers in the league.

That is not to say that his success this past season was all a product of his environment. Changes to his pitch use and pitch sequencing were just as responsible for his resurgence as a pitcher following Mets front office mishandling that cratered his stock.

In 2020, Flexen used his curveball a lot more than he did while pitching with the Mets. During his time on the major league roster from 2017 until 2019, the right-hander used his curveball 6.1% of the time. Despite having one of the higher spin rates for curveballs in the league, it was not an effective pitch, with batters hitting over .500 against it. In Korea, Flexen roughly doubled his curveball usage, throwing the pitch 12.6% of the time. Unlike Major League hitters, KBO batters struggled against the pitch, hitting .179 against it. The pitch averaged 76 MPH, which was slightly slower than the curveballs thrown by other foreign pitchers but was roughly 5 MPH faster than Korean starters.

In recent years, the curveball has undergone a renaissance in the KBO. A commonly thrown pitch— the third most common in the league, behind fastballs and sliders— the importance of curveballs has skyrocketed since the “flyball revolution” began in South Korea. According to Son Hyuk, former manager of the Kiwoom Heroes and a pitching coach before that, “As pitchers are feeling the pressure in a season that has seen an unusually high number of big hits, they’ve been actively learning to command a high fastball. The first step to mastering a high fastball? Nailing a curveball.”

In the United States, Flexen had a 47.2% zone percentage. In Korea, that rate fell by nearly ten percentage points, to 37.7%, as he utilized his fastball/curveball combination to set up and strike out hitters with pitches above and below the strike zone. When thrown in the upper third of the zone or above it, batters hit .117 against Flexen’s fastball. When thrown in the bottom third of the zone or below it, batters hit .122 against his curveball. His slider was also effective when used in a similar manner, with batters hitting .172 against it.

The inability for hitters to square up on his fastball/curveball combo has benefited the right-hander immensely. With the Mets, he had an identical 39.3% groundball and 39.3% flyball rate. The large amount of balls that MLB hitters put in the air had devastating consequences for Flexen. Over the course of his 68.0 major league innings thrown from 2017 to 2019, he allowed 14 home runs. While much of that sample can be considered somewhat unreliable because it came in bursts and spurts while pitching different roles, his numbers were just as poor in Triple-A, where he served as a regular member of the Las Vegas 51s and Syracuse Mets pitching rotations. In 2018, while pitching for the 51s, he allowed 11 home runs in 92.0 innings. In 2019, while pitching for the Syracuse Mets, he allowed 11 home runs in 78.2 innings. In his 116.2 innings with the Bears this past season, Flexen allowed just 6 home runs- and one additional one in the Korea Series. His groundball rate rose to 52.3%, while his flyball rate shrunk to 25.0%.

Just 26-years-old, Flexen has many options available to him. While Flexen was operating under a salary cap because he was a league rookie in 2020, he will not be considered a rookie in 2021 and will be eligible to sign a for more money and for a longer period of time should he and any other Korean teams agree to terms. His success in the KBO has certainly been noticed by NPB scouts, who may also be willing to sign Flexen. His success in the league, coupled with his age and former prospect status has certainly not escaped the notice of MLB scouts and executives, and the right-hander may have pitched his way into a more guaranteed contract than a minor league gamble.