How Will Walter Pennington Translate to Major League Baseball?

Walter Pennington is everyone's favorite solution for a struggling bullpen, but can his stuff play in MLB?

If you listen to the show, you know that we’ve been big Walter Pennington fans since before the season began. His performance in spring training and the first half of the AAA season did nothing to dampen our spirits about this young, left handed relief prospect. In 53.2 IP in AAA this season, Pennington has a 2.35 ERA, 3.26 FIP, and 2.98 xFIP with a 35.3% strikeout rate and 8.8% walk rate (stats as of Sunday, July 7).

That’s real good.

But some have questioned whether or not Pennington’s stuff will play against major league hitters. Preston Farr, the editor-in-chief of Farm to Fountains, posted a tweet after Pennington’s MLB debut pointing out that in the very small sample of his MLB debut, which consisted of only eight pitches, Pennington’s Stuff+ rating was only 68.

While that number can’t really be taken to mean anything (as it’s in an eight-pitch sample), Farr’s broader point, that Pennington doesn’t have the overwhelming stuff associated with can’t-miss relievers, is one to consider.

So, I considered it. I dug into Pennington’s AAA Statcast data and Prospects Live’s AAA Stuff+ model to get insight into his stuff. I then compared that information to pitchers in Major League Baseball. What I found broadly supports the notion that Pennington probably doesn’t have even average stuff, which begs an even tougher question …

How has he been so good in AAA?

First, let’s understand Pennington’s arsenal

In 2024, Walter Pennington has thrown five pitches, three far more than the others. Here’s the breakdown:

As you can see, Pennington throws his slider more than 50% of the time. It’s a slightly below average pitch even relative to other AAA sliders based on the Stuff+ model developed by Prospects Live.* This is where things start getting confusing. Pennington’s slider is a slightly below average pitch, and yet, he gets a swing-and-miss (whiff) on it nearly half the time (48.1%). It is, in fact, tied for the 57th most whiffed pitch in AAA with Carson Whisenhunt’s 70-grade changeup.**

And it’s not like hitters destroy the slider when they’re not missing it. The numbers against it are minuscule. Hitters hit .168 with a .311 slugging against it, and that’s getting lucky. The expected numbers against it are even better (.149/.218).

So, the slider is already perplexing because he gets much better results on it than the Stuff+ indicates he should. Yes, he locates it pretty well—103 Location+ rating—but that’s only a little above average.

One explanation is that his slider gets so much swing and miss because hitters are worried about other pitches. This doesn’t hold much water because hitters know he throws the slider more than any other pitch by far. But there could be something to the notion that his arsenal works well together. His cutter grades out as well above average in Stuff+ at 109. His sinker grades out at an eye popping 135, a HUGE improvement for that pitch over last season.

This is another anomaly of Pennington. In terms of stuff, he throws his third best pitch the most, his second best pitch the second most, and his best pitch the third most. And this probably isn’t a mistake. With previous Royals pitching player development people, I might assume they just didn’t understand what his best pitch mix is. This player development regime has earned the benefit of believing they know what they’re doing.

But it’s important to remember. All of his Stuff+ ratings that we’re using come from AAA so his ratings will be relative to other AAA pitchers, not to MLB pitchers. And the difference between AAA pitchers and MLB pitchers is HUGE.

After examining his arsenal, I looked for players who resemble Pennington to understand how he might transition, but I leveled the playing field a little. Rather than look for pitchers with similar Stuff+ ratings (as we know those can’t really be compared across levels), I looked for pitchers with similar arsenals, velocities, and movements to see if I could find good comparisons.

*The Prospects Live AAA Stuff+ model app hasn’t been updated since April 29 so the data is limited. That said, it’s more than we have to go on from his MLB debut.

**Whisenhunt is the Giants number one prospect, and the 58th prospect overall, according to MLB Pipeline.

Some player comps

After digging into virtually every left handed MLB pitcher’s pitch data, it became clear that finding comps for Pennington wasn’t going to be a clean fit. Like Pennington, there are slider-first, left-handed pitchers out there, but most of them have more movement or velocity on their sliders. And none that I could find also throw a cutter as their second pitch.

So, I’m going to give you two player comps that kind of fit Pennington, and then I’ll explain what I like and don’t like about each one and how I see Pennington resembling (and not resembling) them.

1) John King, 29-year-old LHP, 6’2”, 215 lbs, St. Louis Cardinals
King is the number one comp for a reason. His slider is the one that resembles Pennington’s the most. Pennington’s slider has 40.7 inches of downward movement and 2.4 inches of glove-side movement at an average of 82.8 MPH. King’s has 40.7 inches of downward movement and 2.1 inches of glove-side movement at 83 MPH. Very similar.

King’s slider grades out at 81 on the Statcast Stuff+ model, but he doesn’t use it as his number one pitch. In fact, it’s his third most frequently thrown pitch at just 12%. His sinker, which averages 93.3 MPH and rates as a 79 in Stuff+, is his primary pitch at 57% usage. Pennington’s sinker probably won’t grade out that high, as it has significantly less movement and far less velocity (though it does grade high on PL’s model … I’m not sure why … maybe a shortage of good sinkers in AAA). King throws a changeup as his second pitch.

Like Pennington, King generates a lot of ground balls as a means to success with a 60.2% ground ball rate. He utilizes his sinker-changeup combo to get righties to ground out and his sinker-slider combo to get lefties to ground out. He’s having the best year of his career thanks to an improved (and extremely good) walk rate and his typically excellent home run prevention with a 2.78 ERA and 1.14 WHIP in 32.1 IP.

I’d be ready to call Pennington the next John King if not for the difference in how the arsenal is deployed and the fact that King’s sinker is so much better than Pennington’s. In terms of command, I think they’ll be roughly the same, as King also has excellent command (a 104 Location+ rating for all of his pitches).

2) Andrew Heaney, 33-year-old LHP, 6’2”, 200 lbs, Texas Rangers

Heaney isn’t a reliever, but his slider looks a lot like Pennington’s, and he throws it more than King throws his. It gets 41.4 inches of vertical movement and three inches of glove-side movement at 82 MPH, and he throws it 28.6% of the time—his second most thrown pitch. Hitters have a .322 xwOBA against it this season, which isn’t great.

That’s about all Heaney and Pennington have in common, though. Heaney technically has a cutter, but he’s only thrown it three times all season. He doesn’t throw a sinker. And he doesn’t have the same kind of ground-ball profile that Pennington does.

He gets by with three well below average pitches, good command, and a lot of savvy … which may ultimately be Pennington’s model.

So, what will Pennington be?

This question is so hard to answer for two reasons.

1) There aren’t enough good comps. I looked at every major league pitcher with slider usage anywhere near Pennington’s and couldn’t find one who throws a cutter as his second pitch. Guys with sliders like his don’t throw them that much. And guys who throw sliders as much as him (virtually all of whom have “better” sliders than him) don’t throw cutters as much as he does. His pitch mix and arsenal are weird.

2) Pennington’s success contrasts his pitch data. If someone just showed me Pennington’s pitch data, I’d say “There’s a guy struggling to get out of AA.” A slider with these characteristics shouldn’t have a 48.1% whiff rate even in AAA. A guy with his arsenal shouldn’t have a 35.3% strikeout rate even in AAA. And no pitcher should throw their third best pitch more than 50% of the time.

Seeing Pennington dominate in AAA is like watching a dog flap his arms and fly. It makes no sense … but it’s awesome!

Sure, incredible command might explain a lot of this. The Location+ ratings from the Prospects Live are good—slider (103), cutter (108), sinker (124) … how is he not throwing his sinker more?!?!?! I digress.

The rest must be some combination of deception, tunneling, pitchability, ect., elements that are difficult to quantify and for which there is no public data. That’s the only explanation I can think of that adequately justifies his AAA success.

What does that all mean for his transition to MLB? Start with John King because I think that’s the best comp there is. It’s also possible that Pennington starts to look more like King in terms of pitch mix—becoming a true sinker-slider guy over time. Pennington doesn’t throw as hard as King, but I think there’s a chance he ends up even better. King never had the type of overall success or strikeout numbers in AAA Pennington is putting up this year.

On Twitter, I set expectations for Pennington at ~80-90 IP a season and a 2.75-3.25 ERA. After digging into his data to an insane degree, I’ll probably revise that ERA up just a bit to something like 3.00-3.75 ERA. The innings pitched should stay the same, as Pennington has proven he can go multiple innings.

In the early days of advanced metrics in baseball, there was this idea that to truly understand players you needed to believe their performance and not what their performance should be based on elements like tools, age, and body type. Pennington may represent the flipping of the coin in which we may need to believe his performance rather than what his performance should be based on underlying data.