Taking a Closer Look at Kohl Stewart

There have been mixed feelings about Kohl Stewart this season. Some cite his low ERA as evidence he has been excellent, while other point to his low K-rate and claim he is struggling. Let’s take a closer look to see which is the case.

A Closer Look

The first thing we must consider is Stewart’s age. A prospect’s performance should never be viewed independently of his age: if a player is old for his level, he should be expected to perform well. Conversely, if a player is young for his level, expectations are lower. Kohl Stewart is 19 and is pitching in Single-A. Let’s put that in perspective: he is three years younger than the average pitcher in his league. Last year, Jose Berrios was 19 in Single-A and was likely the youngest pitcher in the league for most of the season. Kohl Stewart is currently the youngest pitcher in the Midwest League. Bottom line, Kohl Stewart is really, really young. As a result, we shouldn’t expect him to dominate. This means that if Stewart is in fact struggling, we shouldn’t worry too much. He can repeat Single-A next year and still be one of the younger pitchers in the league. Furthermore, if Stewart is performing well, we should be much more excited¹.

Now let’s take a look at his numbers: 2.58 ERA (4^th in the league), 1.15 WHIP. You’ve probably seen these numbers. They are very good. One major reason for his dominance is a .268 BABIP (well below the league average of .312). This low BABIP could be a result of poorly hit balls (a good thing) or it could be luck (less good). Uncertainty exists. So let’s look a little deeper at three important percentages with young pitchers: strikeout rate, walk rate and ground ball rate.

As many have noted, Stewart is struggling to strike batters out. He is striking out batters in 17.2% of plate appearances (6.46 K/9), 18% below the league average of 20.3% (7.9 K/9). However, Stewart’s control has been very good: he is walking batters in 7.2% of plate appearances (2.39 BB/9), 19% better than the league average rate of 8.6% (3.3 BB/9). This low walk rate has balanced his lack of strikeouts, allowing Stewart to post a slightly above league-average K/BB ratio of 2.39 (the league is 2.36).

The next area to look at it is Stewart’s ground-ball/fly-ball tendency. According to milb.com, Stewart currently sports a 1.73 GO/AO (air out) ratio. This suggests he is an extreme ground ball pitcher, whereas his 1.24 GB/FB ratio (according to Baseball-Reference) pegs him as more of a moderate ground ball pitcher². Stewart appears to be, at the very least, a moderate ground ball pitcher. The importance of this ground ball tendency is easy to see: opposing hitters have a .079 isolated slugging percentage against Stewart, well below the league average of .119.

What it Means

Just looking at Kohl Stewart’s numbers leads to an interesting conclusion: He appears to be the classic “Twins pitcher”: low K-rate, low BB-rate, high GB-rate. Although it would be nice to see Stewart posting higher strikeout numbers, it can be argued that a low walk rate and a high ground ball tendency are more important at this stage in his career. You rarely see a fly-ball pitcher become a ground-ball pitcher and given the numerous reports raving about Stewart’s “stuff”, optimism remains he can increase his strikeout totals. If he were walking lots of hitters, anxiety about his future would increase.

If Stewart can continue to generate ground balls, limit walks and increase his strikeout rate, he would be poised to become an ace. But even if he doesn’t become a strikeout machine, his excellent control and ground ball inducing abilities at this young age should allow him to be an effective pitcher for years to come.

Last year, the Twins limited Berrios to 103 IP. Stewart has thrown just over 70, so only expect him to log about 30 more before the Twins shut him down. Unless he becomes a dominant strikeout machine before the season ends, the Twins will have an interesting decision about where to start Stewart next spring: do they promote him to Ft. Myers and see how he fares or do you hold him back in Cedar Rapids for a month or two until he displays an improved strikeout ability? This author leans toward favoring the conservative approach—after all, he would still be one of the younger pitchers in the MWL—but hopefully Stewart will finish strong and force the Twins' hand.

¹ That being said, a prospect’s age should not enter the conversation when considering whether or not he should be promoted. A player has mastered a level when he has mastered the level, and that has absolutely nothing to do with how old he is. In other words, although we shouldn’t be scared if Stewart is struggling, we shouldn’t promote him just because he’s young.

² Considering that a higher percentage of fly balls are outs than groundballs, it is surprising that Stewart’s GO/AO ratio is significantly higher than his GB/FB ratio. It is possible that this is the result of an extraordinary number of weakly hit groundballs, or it could be a coincidence.