Digging Deeper into Kyle Zimmer’s breakout in 2020 (and what that means for 2021)

One Royals reliever who had a tremendous 2020 campaign was Kyle Zimmer, the Royals former 1st round pick (and 5th pick overall) in the 2012 MLB Draft. Drafted to be a top-of-the rotation ace out of the University of San Francisco, Zimmer has been plagued by injury in his professional career. Going into 2020, it was expected that Zimmer would flame out eventually, especially after a MLB debut in 2019 which saw him post a 10.80 ERA, 5.78 FIP, and K/BB ratio of 0.98 (comparatively under TWO is not good) in 15 appearances and 18.1 innings of work.

However, even though he had a rough Spring, Zimmer saw a resurgence in 2020 thanks to a change in mechanics, which he tinkered with during the COVID-influenced layoff between Spring Training and Summer Camp, according to the Athletic’s Alec Lewis. The adjustments produced a stellar 2020 line for Zimmer, and he proved to be a reliable option out of the Royals bullpen. In 16 appearances and 23.1 IP, Zimmer posted a 1.57 ERA, 2.37 FIP, 2.60 K/BB ratio, and also produced a 28.6 percent K rate, an 11 percent improvement from the previous year. What was interesting about his improvement was that his velocity went DOWN from 2019 to 2020, as he only averaged 94.1 MPH on his fastball after averaging 96.4 MPH on his four-seamer in 2019. Furthermore, his slider also saw a similar drop in velocity, as it went from 85.2 to 82.9 MPH, according to Baseball Savant data.

Typically, velocity drops, especially for “power pitchers” like Zimmer, are usually warning signs and often result in regression. That proved to be the opposite, however, for Zimmer, as he generated more whiffs and strikeouts on his fastball and slider in 2020, in comparison to 2019. So what’s the difference? And can that growth be sustainable in 2021?

Let’s take a deeper dive into Zimmer’s metrics and see what that could mean for the 29-year-old right-handed reliever next season.


In order to get a better idea of Zimmer’s progression from 2019 to 2020, one has to see how his primary two pitches (his four-seam fastball and slider) became more effective last season. Thus, in the screenshot below, it is important to note how his fastball and slider became better in terms of run value last season. Just to note, for pitchers, negative run value is a GOOD thing, and positive run value is BAD (it is in the inverse for hitters).

The curveball and changeup run value data saw MAJOR improvement on a run value per 100 pitches basis (RV/100) as his curve went from 4.1 RV/100 to -0.4, and his change went from 6.4 RV/100 to -5.2. However, I don’t want to focus too much on those pitches because their overall run values aren’t all that major (one run difference for curve; two run difference for change), and he didn’t throw those pitches all that much, though he did see a 5.6 percent spike in curveball usage from 2019 to 2020.

The real pitches to focus on though are his fastball and slider, as those were his two primary pitches last year, as evidenced by their 48 percent and 34.3 percent usage rates, respectively. What’s interesting about those two pitches is that he adjusted his usage on them from 2019 to 2020, as he threw the fastball 13.1 percent less and his slider 8.6 percent more. And yet, despite the drastic change in repertoire, Zimmer saw substantial gains on a run value difference end, as he improved by 10 runs from 2019 to 2020 on his fastball, and five runs from 2019 to 2020 on his slider. That’s pretty remarkable and explains how his ERA went from ridiculously bad in 2019 to ridiculously good in 2020 in a similar sample size (only a five inning difference roughly from 2020 to 2019).

So, Zimmer’s fastball and slider were much better in 2020, but how? In order to get a better sense of how they were better, I wanted to see some overall metrics, specifically regarding plate discipline, which give a better sense of how Zimmer was generating strikes in 2020. I used Alex Chamberlain’s Pitch Leaderboard Tableau and garnered plate discipline data on Zimmer from 2019 to 2020 in order to see how each of his pitches fared on a strike-generating end over the past two years. That data is displayed in the image below:

So what’s interesting is that his slider has remained pretty stable over the past two years. He actually generated a higher CSW (36.6 percent to 32.8 percent), swinging strike rate (24.1 percent to 19.5 percent), and put away rate (36.8 percent to 32.0 percent) in 2019 than in 2020, respectively. Thus, based on the plate discipline data above, it can be safely said that his slider could be a dependable pitch for 2021, especially since it has been relatively consistent the past two seasons.

The fastball on the other hand is a different story. His fastball was a pretty mediocre pitch in 2019. The 22.9 percent CSW (called strike plus whiff rate) and 8.3 percent swinging strike rate on his fastball in 2019 ranked as the third-worst pitches in their respective categories over the past two years. Furthermore, his 31.3 put away rate was only better than his changeup the past two years, which has only been thrown 35 times over the past two years (not exactly a fair sample).

That being said, his fastball suddenly went from a quite hittable pitch in 2019 to one of Zimmer’s most dependable options in 2020. His 34.1 percent CSW on the fastball was the highest mark in this category from his pitch arsenal in 2020, and his put away rate on his 2020 fastball was nearly double the rate on the same pitch in 2019. And again, Zimmer did this despite a dip in velocity from 2019 to 2020.

Hence, the fastball may be the key here to projecting what 2021 may be like for Zimmer. Royals fans can see from the run value and plate discipline metrics that his fastball was significantly better despite the dip in velocity. But how did that improvement specifically happen?

Well, let’s take a look into some charts and videos of Zimmer’s fastball in 2019 and 2020, and how that correlated to success.


In order to get a better idea of how Zimmer located his fastball, I created heatmaps on his fastball via Baseball Savant’s Illustrator. Let’s see what the contour of his fastball looked like in 2019:

While Zimmer certainly pumped the heat on his fastball in 2019, his fastball kind of sat in the middle of the zone, as evidenced in the chart above. Hence, it is not surprising that hitters posted a xwOBA (expected weighted on base average) of .460 on the pitch in 2019, as hitters knew the fastball was coming in the heart of the strike zones on a regular basis. Here’s Hanser Alberto (now with the Royals) driving a 98 MPH Zimmer fastball into the gap back in 2019 at the K..

98 MPH is pretty fast. However, 98 MPH down the middle and with little to no movement will be eaten for easy base hits all day by Major League hitters, which was the case for Zimmer frequently two years ago.

Now, let’s take a look at his fastball contour from 2020:

In the heatmap above, Zimmer does a much better job locating it on the edges of the strike zone. And consequently, hitters were less effective overall on the pitch in 2020, as they only posted a xwOBA of .275, a near 200 point difference from 2019. Here’s a clip of Zimmer hitting that dark red spot in the chart above against Cubs hitter Albert Almora at the K last season:

Zimmer’s fastball here is FIVE MPH slower than his 98 MPH one in the previous GIF. And yet, what these two GIFs demonstrate is that it’s not necessarily about speed with the fastball with Zimmer. Rather, what’s more important is his command with the pitch.

In 2019, Zimmer wasn’t commanding his fastball properly, and even though he was throwing it in the upper 90’s, hitters were raking it because he was leaving it in hittable zones in favorable counts. Zimmer did the inverse in 2020: locating it on the edges in pitcher-friendly counts. And thus, instead of giving up gap hits, he was getting batters out with his fastball, often by strikeout, as evidenced by the clip above and his 35.9 percent K rate on the pitch in 2020.


There will be heightened expectations for Zimmer in 2021, and how his fastball continues to develop could affect his performance. If his fastball continues to be effective command-wise next year, like it was in 2020, then Zimmer could emerge as one of the better relievers in the Royals bullpen, which is already pretty good as it is. His fastball seemed to not only be an effective pitch in itself, but it also seemed to to amplify his secondary pitches, which actually showed flashes of being good in 2019 (but his fastball was so poor, which affected his overall line)

What’s interesting though about Zimmer’s 2020 was that in comparison to other Royals relievers, he wasn’t really put in a lot of high-leverage situations. According to Fangraphs WPA (win probability) data, Zimmer had the lowest gmLI (0.45) of qualified Royals relievers last year. GmLI measures leverage index of when a pitcher enters a game and the higher the gmLI, the higher the pressure of the situation is when a pitcher comes to the mound. Closers and key relievers will have gmLI in the 1.50 to 2.00 range, which shows that the situations Zimmer entered last year tended to be lower-pressure affairs.

Of course, that’s not surprising. Zimmer in 2019 was horrendous and the Royals probably wanted him to focus on just getting innings and finding success after so many years of struggles, especially with injury. Thus, it will be interesting to see if Mike Matheny will put him in more high-leverage situations as a reliever in 2021, or if he will continue to play it safe with Zimmer, especially considering his inconsistent professional history.

Nonetheless, Zimmer could emerge as a dependable reliever for the Royals in 2021, and his fastball could be the key to another 2019 or a repeat of 2020 or something similar. Zimmer is recovering from injury, so his health will be key to watch this Spring, especially in regard to how it has affected his velocity and command.

That being said, if Zimmer is carrying things over from 2020 into Cactus League play, it would not be surprising to see Zimmer emerge as a possible setup man next year, especially if a reliever at the top (Holland, Barlow, Hahn, or Staumont) gets off to a questionable start (like Ian Kennedy last year).

(Photo Credit: Ed Zurga/Getty Images)

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