Examining Zimmer and Brentz’s recent struggles, and what that means for the Royals long-term

The Royals bullpen has been an up and down group in 2021.

Luckily, Scott Barlow has been solid, as he leads all Royals relievers (with 10 or more IP) in fWAR (1.5), according to Fangraphs. Unfortunately, beyond Barlow, this Royals bullpen has not demonstrated much consistency, much to the dismay of the Royals overall as a team.

As a collective, the Royals bullpen ranks 22nd in reliever ERA going into Tuesday’s game against the White Sox. Furthermore, the Royals bullpen hasn’t been helped either by the struggles of Greg Holland and Wade Davis, two Royals legends re-signed and signed in the off-season, respectively, who have accumulated a combined -0.8 fWAR this season.

Two of the most interesting cases though for this Royals bullpen has been Kyle Zimmer and Jake Brentz. Both are under-the-radar relievers who became widely utilized by manager Mike Matheny in 2021, as they found success early on in the season. However, they have hit hard times as of late, especially over the past month, and Royals fans have to wonder what their futures will look like in Kansas City, especially as this club begins the process to prepare for the upcoming 2022 season.

Let’s take a look at what has caused Zimmer and Brentz’s struggles respectively, and whether or not the pair of breakout relievers could bounce back down the stretch in 2021.


Zimmer started to show gains a year ago, as he went from a pitcher who may have been on his way out of Kansas City to one who was a key member to the Royals bullpen during the shortened 2020 season.

In 23 IP during the COVID-affected season, Zimmer posted a 1.56 ERA and posted a 28.6 percent strikeout rate, an 11 percent increase from the previous year. Furthermore, Zimmer also lowered his walk rate from 18.6 percent in 2019 to 11 percent in 2020, a sign that his command was starting to develop after a rough rookie year where he struggled to find the strike zone consistently.

As a result of a strong 2020, Matheny went with Zimmer in key situations on a more frequent basis in 2021. GmLI measures the pressure of a situation when a reliever enters a game, and the higher the gmLI, the more crucial and pressure-filled the situation is for that particular reliever. Let’s take a look at what Zimmer’s gmLI has been this season, according to Fangraphs:

As Royals fans can see, Zimmer has been inserted in more “crucial” situations than any other Royals reliever who has accumulated 10 or more IP. That also includes Barlow, as Zimmer’s gmLI score is 0.6 points higher than the Royals’ best reliever.

That isn’t faint praise. That high of a number means that Matheny has demonstrated a high level of trust in Zimmer, and early on in the year, Zimmer seemed to follow through on that trust in such important situations in the middle to late innings.

In 11 appearances and 13.1 IP in April, Zimmer posted an ERA of 3.38 over the month with 12 strikeouts to five walks. Zimmer was more hampered in May, as he missed some time and only pitched 5.2 innings. However, in June, Zimmer started to really find his groove as a key Royals reliever, as he not only posted a 3.09 ERA over the month, but also struck out 12 in 11.2 IP. While Barlow’s was the Royals’ best reliever, Zimmer arguably became the Royals’ No. 2 guy out of the pen going into the month of July.

Unfortunately, Zimmer has struggled immensely in the month of July, as he not only posted an 8.71 ERA in 12 appearances and 10.1 IP, but he also seen his hits and walks jump over the month as well. Zimmer allowed 12 hits and seven walks in July, and as a result, he posted a high 1.84 WHIP, which was his worst mark in that category for any month this season.

A big issue for Zimmer has been locating his fastball, as his walk rate on the four-seam fastball is 20 percent this season, according to Baseball Savant. This is a stark contrast to his 2020, where he posted a K rate of 36 percent on his four-seam fastball and a walk rate of 18 percent. This year, the four-seamer is only generating a K rate of 14 percent, and has also seen a hard hit rate increase from 35 percent in 2020 to 44 percent in 2021.

In fact, let’s take a look at how he’s commanding the four-seamer this season, according Savant’s heatmap:

As one can see, that doesn’t look too bad at the surface level. It seems like Zimmer is locating it pretty consistently in one part of the strike zone, which is pretty ideal (especially since it is IN the strike zone).

However, let’s take a look at his July-August fastball command:

As Royals fans can see, Zimmer has been all over the place with his fastball over the past month, and that explains why his command has struggled so much as a result. Here’s an example of Zimmer failing to locate his fastball recently, which results in a walk to White Sox nine-hole hitter Danny Mendick at Kauffman Stadium:

This is a far cry from his four-seam fastball command back in April. Here is Zimmer able to command the fastball effectively in a strikeout of Tigers slugger Miguel Cabrera:

Zimmer hasn’t really turned things around in August, as he walked three White Sox batters on Tuesday and failed to get a single out before Matheny pulled him (his ERA jumped from 4.17 to 4.83 as a result). And unfortunately, it’s his control and command of the fastball that continues to be the root of his problems overall.

If Zimmer wants to recapture that April-June self, he will need to find that elite command again on that four-seam fastball, like he showed in 2020 and in the first three month of play this year.

Otherwise, it could be a rough end of the year for the former first-round draft pick.


Unfortunately, for Brentz, the four-seam fastball command also is proving to be an issue that is getting in the way of Brentz’ effectiveness as of late.

Brentz for the most part has been excellent with his slider, as he is generating a 33 percent strikeout rate on the pitch in comparison to a five percent walk rate, according to Baseball Savant. The slider has been an excellent two-strike pitch for Brentz, especially against left-handed hitters. Here’s an example of Brentz’s movement on the slider tying up Tigers left-handed hitter Akil Baddoo, causing him to strikeout looking:

However, the rookie left-hander has failed to be as consistent with his four-seam fastball and that has caused him to struggle as of late, as he posted a 5.40 ERA in 11 appearances an 8.1 IP in the month. While he did strike out 11 hitters and only walked three overall in July, almost all of his walks have come on four-seam fastballs. In fact, Brentz’s last seven at-bats that turned into walks (stemming back to June 29th), they have all finished on four-seam fastballs, as evidenced in the chart below via Savant:

Here’s an example of Brentz unable to locate his plus-velocity four-seam fastball against the Orioles’ Trey Mancini at Kauffman Stadium:

If Royals fans look at his heatmap on four-seam fastballs that he threw for balls in 2021, the red area (signalizing where the throws it the most) is around the location of the GIF above. Even though Brentz is able to pump up the velocity on his four-seam fastball (he averages 96.8 MPH on it), his mechanics cause him to overthrow the pitch, which results in him throwing a lot of balls around the feet of right-handed batters, as evidenced by this heatmap below:

Thus, when it comes to his four-seam fastball, a mechanical tweak for Brentz may be in order, especially if he wants to lower that walk rate from 22 percent on the pitch, which unfortunately, is just too high to be an effective reliever in the late innings consistently.


The future for Brentz and Zimmer should be safe in Kansas City, as they aren’t costing the Royals a whole lot money-wise, and they are under club control until at least 2026. That being said, if they want to be crucial relievers going forward, their four-seam fastball command will need some refinement, as their lack of ability to control and harness their four-seam fastballs has been a major downfall for both of them, especially over the past couple of months.

Right now, I feel better about Brentz than Zimmer, as the latter seems to be a shell of himself, especially after his most recent disastrous outing against the White Sox. Brentz at least can throw in the upper 90s with his fastball, and his slider is truly a “strikeout” pitch. Zimmer has seen his overall strikeout rate decrease 4.2 percent this year, and his fastball velocity hovers around 94.3 MPH, which ranks him in the 67th percentile, according to Baseball Savant. While that isn’t bad, that isn’t 93rd percentile fastball velocity like Brentz, and that means command is going to be a lot more important for Zimmer going forward, especially since Zimmer is going to be 30-years-old next year and doesn’t have the kind of stuff that Brentz possesses.

Nonetheless, the Royals will need an improved bullpen beyond Barlow if they wish to compete in 2022. Any good team needs relievers developed within their own system that can contribute on the cheap. Both Zimmer and Brentz have that profile (cheap relievers) and when things are clicking, they can make the Royals bullpen one of the better ones in the league.

Unfortunately, they are struggling now, and they will continue to regress unless they iron out their respective issues with their four-seam fastball command.

It will be interesting if Cal Eldred and the Royals pitching staff will be able to help them correct this issue over the next couple of months…

Because if they aren’t able to, that could mean even further declines from Zimmer and Brentz not just in 2021, but perhaps 2022 as well.

And that would only sink the Royals’ chances of competing next year even further.

Photo Credit: Ed Zurga/Getty Images

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