Royals Deep Dives: Angel Zerpa's transformation into a solid reliever. – Farm to Fountains

Welcome back to another iteration of the deep dive series. This week we will focus on the “arm barn” for the Kansas City Royals. More specifically, we will focus on Angel Zerpa’s year and his role change to middle reliever. So, how has Zerpa done? Let’s dive into it.

As always, we start off with the basic numbers. Zerpa is sporting an ERA of 3.56 to go along with a K/9 of 8.01, BB/9 of 2.97, and a HR/9 of 0.89. On paper that is pretty solid and compares well to other middle relievers. While those numbers are solid once we dive deeper we will see some major concerns with Zerpa.

To preface this relievers are always volatile so one bad outing skews a ton of things. With that, Zerpa is coming off his worst month of the season in June. He had a 10.29 ERA, walked 10.3% of batters faced, and had a 2.29 WHIP. All of this was in seven innings pitched as well, so he fell out of the circle of trust a little bit. It was a tale of two months for Zerpa as June was terrible, but in May he only gave up one earned run.

What changed from May to June for Zerpa? Well, he was a tad more unlucky in June. In June, he had three outings in which he gave up earned runs. One of those outings was a dud against the Padres where he didn’t get an out while giving up two earned. Another outing was against the Rangers when he gave up five earned runs. The final outing of June against the Guardians is the last earned run he gave up. Obviously, the Rangers outing spiked everything in his numbers and in June he had eight solid outings out of eleven.

That begs the question of what the deeper numbers say about Zerpa. Let’s just say he has some of the more polarizing numbers of any reliever this writer has covered. As per Baseball Savant, Zerpa is really good at two things. His fastball velocity and groundball rate, which land in the 77th and 96th percentile respectively. That is all he is good at percentile wise.

That means he is bad at getting swings and misses, doesn’t K batters, gives up hard hits, has terrible extension and many other things. There are a ton of holes in Zerpa’s game at the moment. In terms of chase%, whiff%, average exit velo, and extension he ranks in the bottom 5% of the league in all of those. This means that his groundball rate is saving him right now.

One of the biggest switches coming into the year for Zerpa was making his sinker his primary pitch. That and with the Royals moving him to the bullpen he upped his velocity on it. His pitch mix of a sinker, slider, changeup and 4-seam has helped him produce grounders at a high rate. The reason that has helped him is because his hard hit rate is 47.9%. This means if he is hit hard it is getting drilled into the ground more often than not.

That is both problematic and promising for Zerpa. On one hand he gets a ton of grounders, but on the other he gives up a ton of hard contact. Let’s also throw in that his average launch angle is 4.5 on the year which is great given his year would be a lot worse if it wasn’t that low. These numbers also explain why the expected numbers dislike him a ton. Zerpa has an opposing expected batting average of .272, expected slugging of .452, and a xwOBA of .340.

There is a silver lining in these numbers, however, as Zerpa’s game is favored by a couple other numbers. One of those numbers, and one this writer doesn’t use often, is SIERA (skill-interactive ERA). SIERA is like xFIP, but it considers the kind of balls in play. Zerpa has a SIERA of 3.42, which is around average and is a career number for Zerpa. That is good and his xFIP- is 88, meaning he has an xFIP that is 12% better than league average.

On the surface he is a solid middle man, but deeper there are concerns with some numbers and other numbers don’t follow that. Let’s throw more into the proverbial fire as Zerpa’s stuff+ numbers are also good. He has a stuff+ of 90, location+ of 104, and a pitching+ of 102. All of these numbers show us that Zerpa has a ton of narrative choices for fans to pick from, but the biggest concerns are in some of the simpler numbers.

If you are a reliever in the MLB you need to have some swing and miss stuff. This year Zerpa does not have that. His whiff% is 13.5% and his chase rate is 20.3% on the year. Zerpa isn’t a huge swing and miss pitcher, but this year is extreme. Only one of his pitches as a whiff% above 20% in his changeup, a pitch that opponents are slugging at .750. His slider, which on paper is his best pitch, has a whiff% of 14.5% which is down nearly 10% from last year.

As it stands right now, Zerpa is a solid reliever. The deeper numbers give conflicting narratives and the surface numbers tell more of the story of Zerpa. The biggest thing he needs to work on is his swing and miss stuff or his extension of 5.6, which is in the first percentile of the league. Zerpa is a groundball merchant with solid stuff he just needs to get that swing and miss if he is going to be an above average reliever. Oh, did I forget to mention he is still 24 and the Royals seem to be done with making him Ryan Yarbrough 2.0. Give him a little more time.

Photo credit: (Denny Medley-USA TODAY Sports)