Written by: Justin Choi
Follow him on Twitter: @justinochoi
Follow Prospects Worldwide on Twitter: @ProspectsWorldW
Over six starts this season, 19-year old rookie Hyeong-Jun So of the KT Wiz has a 5.35 ERA. In 33 innings, he’s only struck out 13 while walking 9. Already, you may be questioning why I’m dedicating an entire article to him. After all, Min-ho Lee, the LG Twins’ own rookie pitcher, is clearly having a better season – a 1.16 ERA and respectable 3.28 FIP in 23 innings.
There also aren’t any pitches within So’s repertoire that stand out. He relies on a fastball and sinker that are thrown in the low 90-mph range, a slider, a changeup, and an infrequent curveball. According to Statiz’s pitch run value page, only his fastball is noticeably in the green; the rest are mediocre pitches that, in part, contributed to a 5+ ERA. He’s not exactly GIF material.
Despite my two paragraphs of negativity (which is anything but, and I’ll explain why later), So is much more interesting than you think. To wit: here’s a record that he set early on in the season.
In addition, he’s pitched at least 5 innings in 6 starts, a feat that hasn’t been achieved since Hyun-jin Ryu’s rookie season (2.23 ERA in 201 innings!). While it’s almost certain that So won’t have a season of that caliber, these two facts show how consistent he is for a 19-year old rookie.
OK, you probably aren’t sold on So yet. That’s fine, because I haven’t even got to my main point. I’d first like to present the change in pitch% of his primary three pitches – a fastball, sinker, and changeup.
In the span of 6 games, the percentages have fluctuated wildly – what was once So’s least thrown pitch, the changeup, is now his main weapon his choice. This in itself, however, isn’t what’s interesting. It’s common for rookie pitchers to toy around with their pitches, seeing what works and what doesn’t.
What stands out is the fact that these changes are reactionary; in other words, So is changing his pitch% based on the results of his previous games.
Consider the drastic increase in changeup% and contrasting decrease in sinker% between Game 4 and Game 5. What motivated So to switch things up? During Game 4, played on May 28th, hitters slugged .875 against his sinker, which he threw 63.3% of the time. On the other hand, they slugged just .333 against his changeup. That doesn’t look too impressive, but his slider fared worse: 1.000 SLG% against. Out of So’s breaking and off-speed pitches, his changeup was clearly a shining gem.
As a result, he made an adjustment in Game 5. He slashed his sinker usage in half – 63.3% to 33.3% – to make room for extra changeups. The result: 7 shutout innings with 2 strikeouts, the best outing of So’s short career. Soft contact generated by the changeups allowed him to cruise through the Doosan Bears lineup:
Whether the changes in pitch usage occurred at his discretion or not is unknown, but it’s a clear sign that Hyeong-jun So is paying attention, a fantastic habit for a young player. A one-game adjustment leading to success is just as significant as a low ERA and/or high strikeout totals.
It’s a philosophy that KT Wiz manager Kang-Chul Lee shares too:
“This year’s results are important, but I wish he (So) can gain experience and stay healthy as a member of the rotation. That’s a process necessary for him to become successful in the following years.”
Kang-Chul Lee (이강철) – MK Sports
Another aspect of So’s growth as a pitcher is his failures. Hyeong-jun So took the mound for his sixth career start with a similar game plan – dominate with soft contact from his changeup. Unfortunately, KIA Tigers hitters came prepared:
| Date | Changeup% | SLG% |
| 06-03 | 41.7% | .000 |
| 06-09 | 39.2% | .750 |
After the dust cleared, So had a final pitching line of 5 IP, 3 ER, 4 H, 2 BB, and 3 K. It’s a disappointing performance, but once again, Kang-Chul Lee defended his young pitcher, telling the media that he pitched well. I wholeheartedly agree – even though his original plan failed, So remained calm and fought his way to five innings.
Hyeong-jun So’s next start arrives in a couple of days. What other changes can he incorporate? I’d suggest throwing his fastball more often. By pitch value/100, which measures the run contribution of a certain pitch per 100 thrown, So’s 3.80 beats Chang-mo Koo’s 3.23! Is this due to a small sample size? Of course, but it wouldn’t hurt to frequently establish counts with a fastball that sits comfortably at 90 mph. If hitters destroy it, all he has to do is go back to the drawing board; as a rookie pitcher, there’s less pressure.
We gravitate towards the super rookies, the pitchers who arrive on the baseball scene with electric stuff, to the point where we forget about the effort that goes into pitching, the constant learning and adapting. It’s the effort that defines Hyeong-jun So, that makes him fun to watch.