One of the most promising stories of 2020 and in the first couple of months of 2021 was the emergence of Kyle Zimmer as a reliable option in the Royals bullpen.

The fifth overall pick in the 2012 draft out of the University of San Francisco, Zimmer was expected to be a cornerstone of the Royals pitching staff when he was selected. Unfortunately, despite showing promising stuff and results in the Minors, Zimmer constantly battled injury throughout his professional career, and it looked like after the 2018 season that Zimmer would never pitch in a MLB, let alone Royals, uniform.

Thankfully for him and the Royals, Zimmer did earn a spot on the 2019 Royals team, and made 15 appearances and pitched 18.1 innings out of the bullpen. Though the results weren’t great (10.80 ERA, 5.78 FIP, according to Fangraphs), the fact that he was pitching at the Major League level in general was something to celebrate, especially considering his injury history.

However, Zimmer, who appeared on the verge of perhaps being DFA’d in Spring Training of 2020, really surprised during the shortened 2020 season. He posted a 1.57 ERA and 2.37 FIP in 16 appearances and 23 IP, and showed incredible control and command, something he struggled with in his initial MLB debut. After posting a 0.95 K/BB ratio in 2019, he improved that ratio 2.60 in 2020, which also included a K rate of 28.6 percent, an 11 percent improvement from 2019.

According to Baseball Savant, Zimmer saw the biggest improvement on his fastball in 2020 from 2019. In 2019, Zimmer only posted a whiff rate of 18 percent and a put away rate of 13.9 percent on his four-seam fastball. Hitters in 2019 posted a .457 xwOBA on the fastball, and this double in the GIF below from Roberto Perez of Cleveland that season demonstrates the struggles Zimmer had when it came to commanding the pitch in his rookie debut:

However, in 2020, Zimmer found command on his four-seam fastball, and it pretty much revitalized his career. His whiff and put away rates improved to 28.6 and 29.8 percent, respectively, and hitters only posted a xwOBA of .283 on the four-seam fastball. Zimmer made some mechanical adjustments during the layoff between Spring Training and the start of the MLB season in July that seemed to benefit Zimmer with the pitch. Even though the four-seam fastball had a slower average velocity (94.1 MPH) in 2020 in comparison to 2019 (96.4 MPH), it was far more effective due to better control and command from those mechanical changes.

Here’s an example of Zimmer demonstrating that supreme fastball command by painting the corner to strike out Milwaukee’s Jace Peterson looking in 2020:

So, after a solid 2020, the expectations were higher for Zimmer in 2021, as he was expected to be a key contributor in the late innings with Scott Barlow, Greg Holland, and Josh Staumont, who all had stellar 2020 campaigns as well. And early on, Zimmer seemed to be living up to those expectations.

According to Fangraphs splits, he posted a 3.46 FIP in April and followed that up with a 2.63 FIP in May, though he only pitched 5.2 innings in May due to injury. However, in June, his FIP increased to 3.76, and then ballooned to 7.03 in July across 10.1 IP. He landed on the IL again in the beginning of August, and when he did pitch again later in the month (three total innings), he was absolutely brutal, as he posted an 11.83 FIP.

Hence, after being one of the Royals’ more reliable relievers during the early months of the 2021 campaign, Zimmer has been pretty much regulated to mop up duty, with the game usually out of reach when he comes in.

With Zimmer about to turn 30 on September 13th, the Royals are in a bit of a dilemma with the former first round pick. Can he bounce back in September and be a reliable option in 2022, like he was in 2020 and the beginning of 2021? Or is the Zimmer who Royals fans have seen lately the “real” Zimmer, who doesn’t offer much to this bullpen beyond relief depth?

---

To start off in my analysis, I immediately looked at his four-seam fastball metrics to see how it fared this season in comparison to 2020. As expected, Zimmer has seen some regression on the pitch, and thus, it’s not a surprise that Zimmer has regressed overall from a year ago.

Zimmer throws his four-seam fastball 49.2 percent of the time, according to Baseball Savant, which is 1.2 percent higher than last season. Unfortunately, despite using it more, it hasn’t been as effective this season. His whiff rate on the pitch is only 15.8 percent and his put away rate is 12.6 percent, which is actually lower than his 2019 rates in those categories. While his xwOBA of .358 is not as bad as his 2019 mark, it is still 75 points higher than 2020, which is not a good thing.

An issue for Zimmer has not necessarily been the longball with his four-seam fastball this season. He has only given up two home runs on the pitch, which is actually half the amount of home runs he has given up with slider (four home runs allowed). That being said, he has struggled to command the pitch, especially since giving up his last home run on the pitch, which came on July 10th against Cleveland’s Oscar Mercado.

Here’s a look at how his fastball has fared since July 10th:

As one can see, hitters are only hitting .258 on the pitch with an xBA of .246. Unfortunately, he has walked 17 batters on the pitch, and his walk rate of 21 percent is seven percent higher than his K rate on the pitch, which is near 2019 levels for Zimmer.

Below is an example of Zimmer just being unable to control the four-seamer, and it produces an easy walk on a 3-1 count to the White Sox’s Andrew Vaughn:

To compare, let’s take a look at his four-seam fastball performance from April to June, which was his best stretch of the season before things went south for Zimmer:

As one can see, the walk to strikeout numbers don’t look great, as he has the same amount of walks to strikeouts (a 1.00 K/BB ratio, which is subpar). However, he was able to limit hard contact, as he only gave up two barrels and only one extra base hit in 47 plate appearances where he used his four-seam fastball. Furthermore, the fact that his xBA (.188) and his BA (.189) are near similar was also a promising sign that he wasn’t just getting lucky on the four-seamer over that time span, but rather locating it effectively.

Here’s an example of Zimmer pinpointing it well, generating a weak groundball from the Red Sox’s Marwin Gonzalez, which induces an inning-ending double play:

Therefore, based on this analysis, how Zimmer’s fastball fares down the stretch could be a key indicator of whether he will be able to rebound down the stretch of the 2021 season. His slider is actually generating a better whiff and put away rate (36.6 and 21.7 percent) in comparison to 2020, and though his curve ball whiff and put away rates are slightly lower this season (26.5 and 18.4 percent) in comparison to last year (30.8 and 22.2 percent), they are still respectable for a third pitch.

Zimmer has the secondary stuff to be a good reliever again for the Royals. But he will need to fix his command and control on his four-seam fastball to make at least a decent pitch, even if it may not be the elite one that it was back in 2020.

---

David Lesky of Inside the Crown posted this on Twitter recently, and it definitely is food for thought when it comes to examining Zimmer’s struggles over the past couple of months:

Hey, another walk from Zimmer. That’s his 15th (against 13 strikeouts) in 14.2 innings since the sticky stuff ban. I don’t know if there’s correlation, but the timing is there.

— David Lesky (@DBLesky) September 1, 2021

It’s an interesting correlation, and it could explain why he’s missing so much with his fastball in August and July in comparison to April through June. However, I did check out his active spin table from 2020 and 2021, just to see if there was any deviation:

As one can see from above, it doesn’t seem like there is much of a spin difference (which sticky stuff primarily would affect) from 2020. However, I did find his plate discipline metrics indicate a concerning regression in one category that contributes to his higher walk rate (14.4 percent, which ranks in the bottom four percent of the league, according to Savant).

Notice the 9.2 percent regression in chase rate, which is huge for a reliever coming into big situations. Hitters have been less likely to swing out of the strike zone, or just in general, against Zimmer, as hitters are swinging overall three percent less in comparison to 2020.

Now, that could mean a variety of things: better scouting on Zimmer, Zimmer tipping certain pitches, etc. However, his chase rate ranks in the 16th percentile in baseball, according to Savant. Considering his dicey control on his four-seam fastball lately, hitters are going to continue to be patient against Zimmer and swing less and less over the next month of play, and thus, those rates could regress further if his control doesn’t improve.

I am sure there are a lot of contributing factors to Zimmer’s struggles this year, and his injury issues probably haven’t helped, as he has had two 10-Day IL stints this season in addition to the 60-Day one he was placed on back in October after the conclusion of the 2020 season. I think that definitely has affected his rhythm, and it’s possible that there is something lingering that is affecting his mechanics, especially when he throws his fastball.

Whatever the reason, it would be nice to see Zimmer finish the year strong, especially with the Royals trying to figure out who will be depended upon out of the bullpen in 2022. Domingo Tapia and Joel Payamps have emerged as solid options after being acquired during the season. If they continue to do well, and if Tyler Zuber can solve his own control issues at the MLB level, then it will be hard to find a spot for Zimmer, especially if his control doesn’t get better in September.

Zimmer has showed when he’s on, he can be one of the Royals’ best relievers.

But he needs to figure out these issues with his fastball…

If he doesn’t, it wouldn’t be surprising to see him perhaps pitching somewhere else in 2022.

Photo Credit: Jay Biggerstaff-USA TODAY Sports