The Royals’ pitching staff has been a strength of this team in 2025. According to Fangraphs, the Royals rank 8th in pitching fWAR with an 8.3 mark, 6th in team ERA at 3.44, and 11th in FIP at 3.82.
The solid pitching results have been a big reason why the Royals’ record isn’t worse than 34-38. Based on their -28 run differential, Kansas City has an xW-L record of 32-40. The pitching has helped them win those close games that they may have lost this season.
However, June has not been kind to the Royals’ pitching staff, especially the bullpen.
While it’s not as glaring an issue as the Kansas City offense, the bullpen has significantly regressed this month, according to Fangraphs. In June, they rank 27th in relief pitcher fWAR with a -0.1 mark, 18th in reliever ERA at 3.97, and 20th in FIP at 4.14.
With the offense struggling and the relievers failing to find consistency, it is not a surprise that the Royals are 3-10 this month.
Thus, is the Royals’ bullpen just going through a tough stretch? Or should fans be worried about this group hitting significant regression in the summer months?
Let’s look at three key Royals relievers who have produced eye-popping numbers (both good and bad) in June (via TJ Stats) and what we can take away from those metrics regarding their outlook for the remainder of the season.
Angel Zerpa (9.2 IP, 0.00 ERA, 1.83 FIP, 0.62 WHIP, 25.7% K-BB%)
According to Fangraphs, since June 1st, Zerpa has been the Royals’ most valuable reliever.
In seven outings and 9.2 innings of work, he has a 0.00 ERA, a 1.83 FIP, and 0.3 fWAR. Those marks led all Royals relievers over those two weeks. As a result of this solid stretch, Zerpa has lowered his ERA to 3.73 for the season in 31.1 innings. That’s encouraging to see, especially after a lackluster start from him at the beginning of the season.
The strikeouts have increased significantly for Zerpa in June, as he is sporting a 34.3% K rate. The Venezuelan lefty has always done a good job of limiting walks, and his walk rate of 8.6% has helped him produce an elite K-BB% this month (25.7%). Thus, it’s not a surprise that manager Matt Quatraro has leaned on Zerpa heavily in June, as his 9.2 IP mark leads all Royals relievers this month.
A big key for Zerpa has been his four-seamer, which has been his best swing-and-miss pitch in June.
He is throwing the pitch 21.7% of the time, 20.2% less than his sinker. However, his four-seamer is averaging a chase of 27.8% and 44.4% whiff rate this month, which are much better marks in those categories than what we’re seeing from his sinker.
Here’s how Zerpa’s four-seamer has fared in June via his TJ Stats heatmap chart and data set.
Against lefties, he’s producing a whiff rate of 50% and a CSW of 33.3% with his four-seamer, even though he only throws it 8.5% of the time.. Surprisingly, he’s leaving that pitch more in the zone against lefties, as evidenced by a 33.3% Heart%. Thus, it’s not surprising that the four-seamer is giving up an xwOBACON of 0.528, which is high.
Zerpa’s progress with his four-seam fastball against right-handed hitters has been the real key to his June success.
His four-seam usage rate against righties is 32.5%, and he’s producing a 25.9% CSW and 42.9% whiff rate with the pitch. Zerpa has demonstrated much better command with the four-seamer against righties, as his Heart% is only 18.5% and his Shadow% is 55.6%. The data and his corresponding pitch heatmap chart against right-handed hitters show that Zerpa is pinpointing his four-seamer well against right-handed hitters, and it’s producing results this month.
Here’s an example of Zerpa commanding his four-seamer up in the zone against Chicago’s Miguel Vargas to generate a swinging strike.
Zerpa is honing his command and mix a bit this season, with his sinker being his go-to pitch against lefties and his four-seamer his main fastball offering against righties. This prowess, along with his ability to pitch multiple innings, has helped him succeed this month, even with the pitch quality being somewhat suspect, based on TJ Stuff+ data (97 overall TJ Stuff+).
At this point, one could say that Zerpa has probably emerged as the Royals’ most dependable left-handed reliever in the bullpen over Daniel Lynch IV.
The 28-year-old Virginia product sports solid ERA numbers (2.20 this year; 3.18 this month). Conversely, he has demonstrated questionable pitch quality (97 overall TJ Stuff+), a high FIP (6.60 in June), as well as low strikeout (15.4% this month) and K-BB rates (0.0% this month).
Don’t be surprised to see Quatraro utilize Zerpa in more high-leverage opportunities instead of Lynch as the season progresses.
John Schreiber (4.1 IP, 6.23 ERA, 4.23 FIP, 1.85 WHIP, 18.2% K-BB%)
It hasn’t been a good month for the Royals’ setup man. Not only is he posting a 6.23 ERA in 4.1 IP, but he also has a blown save on his record this month, which happened in Sunday’s game against the Athletics, where he gave up a game-tying home run to Athletics catcher Austin Wynns.
Schreiber has been okay this season, though not elite by any means.
His 3.95 ERA is up from last season, and his barrel rate is 10.1%, a 6.2% increase from the previous year. However, his 25.2% K rate is 4.7% higher than a year ago, his xwOBA is 23 points lower than his actual wOBA, and his 3.80 xERA is 15 points lower than his actual ERA.
Thus, Schreiber has been impacted by the big hits this year, and the numbers this month reflect a similar trend.
Even though his ERA is 6.23, he has an 18.1% K rate and hasn’t allowed a single walk. His 1.85 WHIP and .421 xwOBACON demonstrate that hitters have been making a lot of contact against him, which correlates with his high ERA marks in June.
His sinker has been the most interesting pitch of his mix this month. He throws it 30.2% of the time, the highest usage of any of his offerings. He also has a 100 TJ Stuff+, 60 grade, a 69.2% zone rate, and 17.6% whiff rate, all excellent marks.
However, hitters have been torching the pitch when they’ve made contact, as evidenced by his .521 xwOBACON on the sinker this month.
Let’s look at his TJ Stats sinker heatmap data and chart to see if any trends emerge.
Both lefties and righties have done well against the pitch with .360 and .567 xwOBACON marks, respectively. However, righties have done way more damage (207 points better, to be specific) due to Schreiber’s lackluster command with the pitch.
Against righties, he has a Heart% of 50%, which is significantly higher than his 12.5% Heart% against lefties. Even the heatmap shows Schreiber locating his sinker way too much in the middle. It’s not hard for hitters to do damage against such a pitch when it’s grooved in the zone (especially with tailing action that goes even more into the strike zone against righties).
Below is an example from the Cardinals series where Schreiber located effectively against lefty Victor Scott II on June 3rd (resulting in a strikeout), but struggled with sinker command on June 5th against righty Ivan Herrera (which resulted in a base hit).
Surprisingly, he had better velocity against Herrera. That said, his poor command of the pitch resulted in a base knock for the Cardinals’ designated hitter/catcher.
Schreiber has gone through stretches in his Royals career where his command has been spotty, which has frustrated Royals fans, especially when he is in big spots. With Lucas Erceg back as the fireman/8th inning setup man, Schreiber will revert more to medium-leverage situations, which I think will help him get his command, especially with the sinker, back on track.
Carlos Estevez (5.1 IP, 3.38 ERA, 5.88 FIP, 1.69 WHIP, 4.0% K-BB%)
On one end, the overall profile of Estevez has been solid in 2025.
In 30 outings and 30.2 IP, the 32-year-old closer has a 2.05 ERA, 1.17 WHIP, and 19 saves. According to Fangraphs, his save mark is the second-best total in Major League Baseball, behind only San Diego’s Robert Suarez (who has 21). He has also been a fan favorite due to not only his performance, but his post-save celebration.
That said, his rough June shouldn’t be ignored.
While his ERA this month is decent at 3.38, his 5.88 FIP, 1.69 WHIP, and 4.0% K-BB% are all mediocre marks (and that’s being generous). Estevez has still demonstrated solid stuff this month with a 103 overall TJ Stuff+, as well as an ability to flood the strike zone with a 55.7% zone rate. Unfortunately, he’s not generating any chases or whiffs, and that’s affecting his overall lines.
In June, Estevez is only posting a 17.9% chase rate and 17.5% whiff rate. He hasn’t exactly been great in these categories overall this year, as his 22.3% chase and 17.9% whiff rank in the third and fifth percentiles, respectively, according to Savant. However, he’s usually been able to make up for those unimpressive marks with a strong ability to minimize hard contact.
That hasn’t happened in June.
Estevez’s xWOBACON is .550, 182 points higher than his season-long mark in that category. A pitch that has gotten hit hard this month is his four-seamer, as it sports a .591 xwOBACON, the highest of his three offerings.
Here’s what his four-seamer looks like against lefties and righties, via the TJ Stats heatmap data.
As seen in the xwOBACON splits, Estevez has been walloped by both lefties (.571) and righties (.605). How hitters are doing it, though, has been much different.
Against lefties, the Royals’ closer is leaving way too many four-seamers in the heart of the zone, as evidenced by his heatmap (look at all that orange in the middle) and his 37% Heart%. He’s throwing strikes with the four-seamer against lefties (25.9%), but they’re not missing (11.1% whiff).
Against righties, he’s not just giving up a lot of base hits, but he’s been a bit wild with the pitch in June.
His Heart% is only 18.8%, which is good, as it means he’s not just grooving it down the middle (his heatmap shows that). He is also sporting a 16.7% whiff rate with his four-seamer against righties. Conversely, he’s not generating many strikes with the four-seamer against righties, as evidenced by a paltry 12.5% CSW.
Essentially, when he’s not generating whiffs against righties with his four-seamer, he’s giving up base hits, which is not what Royals fans want to see from their closer.
Here’s an example of Estevez making that command mistake against left-handed first baseman Nick Kurtz, who unloads on a 94 MPH four-seamer grooved in the middle on a 2-2 count.
That’s too big of a mistake from Estevez, especially with two strikes and against a rookie hitter.
The damage hasn’t been as severe as it could have been this month for Estevez. After all, his ERA is 2.51 lower than his FIP, which shows that Estevez has skirted by with some batted-ball luck in June.
That said, these kinds of stretches are to be expected with Estevez and his pitching style.
The 32-year-old Dominican pitcher is known more for generating weak contacts and field outs than whiffs and strikeouts. Based on that, it will be typical for the closer to outperform his FIP, especially since FIP values and rates pitchers higher with better strikeout rates.
However, while it’s okay for Estevez not to have a ton of strikeouts, walking batters is a different story and could make rough stretches like this more common this season.
As of Monday, Estevez has a 16% walk rate in June and an overall walk rate of 10.9% in 2025. That has produced not only a 4.0% K-BB% this month, but a 10.9% K-BB% this season. That represents a 7% decrease from last year and a 4.2% decrease from his career average in that latter category.
He is getting away with it now, but he won’t be able to get that kind of luck for long. This month has demonstrated that, and it could go south quickly if the command issues continue for Estevez deeper into the summer.
Royals fans can live with the lack of whiffs and strikeouts from Estevez, even in those crucial spots in the ninth.
The walks, on the other hand? That won’t be tolerated, especially with a pitcher like Erceg, who can come in and slide into that closer role quite easily (as long as he stays healthy). Here’s what Erceg has done in June in two outings since coming off the IL.
Erceg’s stuff is ramping up after a layoff. However, even in that small sample, his chase and whiff metrics have outperformed those of Estevez.
Thus, Carlos will need to get his command and control back if he wants to hold off Erceg, especially once the former A’s pitcher gets fully healthy again (and the TJ Stuff+ gets back to his typical levels).
Photo Credit: AP Photo/Charlie Riedel