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Breaking Down the Cardinals Rotation Swap

An injured Steven Matz creates opportunity for young arms.

MLB: St. Louis Cardinals at Kansas City Royals Jay Biggerstaff-USA TODAY Sports

In case you missed the news yesterday, Steven Matz is heading to the Injured List after straining his lat in his last start, which could potentially be his last start of the year, and Zack Thompson will replace him in the rotation after a string of impressive appearances.

There are a number of implications to break down here and that’s what I want to do today.

Tough break for Matz but it doesn’t change much

This injury couldn’t have come at a worse time for Steven Matz because he was in a groove. After a rough start that saw him get demoted to bullpen, Matz put up a 1.86 ERA, 2.85 FIP, and 3.67 xFIP with a 25.2% strikeout rate and 4.6% walk rate. Those are all outstanding numbers.

I should point out, though, that he was benefiting from a higher than usual 83.9% left on base rate and .233 BABIP while giving up home runs on just 7% of his fly balls.

That’s a lot of numbers so I’ll sum it up by saying that Matz was absolutely pitching well but also had some beneficial luck that boosted his numbers. While this injury interrupted a strong stretch for Steven Matz, it doesn’t really matter. Sure it matters to Matz because no pitcher likes to be injured and it certainly matters to the St. Louis Cardinals as they are now down one of their better pitchers, but this injury doesn’t change anything for next season, which is what really matters at this point.

Among the starters, the Cardinals only have Mikolas and Matz under contract so they have to rebuild most of the rotation next year. They already had to bring in 3 starters (and have stated their intention to do so) and they’re not about to increase that number to 4 because of a lat strain to one of their two starters under contract for next season.

Matz’s spot is safe. He’s had an inconsistent year but currently has an ERA, a FIP, and an xFIP below 4 and has been worth nearly 2 fWAR. His spot in next year’s rotation is secure.

So, while this injury is tough news, it doesn’t change much. Matz will miss some starts this year and that’s it. And that doesn’t even matter because this season is already lost.

There are practically no long terms effects because of this injury, so let’s not read into it too much. The biggest effect is that...

Zack Thompson gets his chance in the rotation

Zack Thompson is another pitcher who has been throwing the ball well recently. And his recent hot streak corresponds with him tweaking his arsenal.

As you can see from the graph above, Thompson has recently started throwing his cutter (which he calls a slider) again. The results since he started throwing the pitch are fantastic but this is where the small sample size really needs to be taken into consideration.

For starters, Zack Thompson made 11 Triple-A appearances this year (9 starts) and was absolutely terrible. There’s no sugarcoating it. The lefty had a whopping 8.65 ERA and 6.78 FIP with a nearly identical 22.9% strikeout rate and 21.8% walk rate. That’s rough.

After that performance, we should be skeptical that Thompson can pitch well in the major league rotation. But let’s go a little bit deeper now and look at pitch mix. This matters because it’s when Thompson really started breaking out his cutter. He only threw the pitch 69 times but it performed well in that limited sample, generating an 81.3 mph average exit velocity and a modest 22.6% whiff rate.

Again, that’s a small sample but at least the pitch played well. The same can’t be said for Thompson’s other pitches but it’s really hard to get a clear picture of their quality since Thompson couldn’t find the strike zone at all in Memphis. That seems to have been an isolated incident, though, because while Thompson hasn’t been a great strike thrower in the major leagues, he has pounded the zone a whole lot more than he did in Triple-A. It was really just a weird Triple-A stint for him and I don’t what to make of it.

Let’s look past that now and arrive at stuff+ models because those provide us further reason to be wary.

Zack Thompson By Stuff Models

Model FB CB CT CH Overall
Model FB CB CT CH Overall
Stuff+ 88 65 79 22 79
PitchingBot 46 67 63 31 50

If you’re unfamiliar with stuff models, let me help you understand this data. Stuff+ is scaled so that 100 is average while PitchingBot operates on the standard 20-80 scouting scale with 50 being average.

As you look at the two models, you can see a huge discrepancy. Stuff+ pretty much hates Zack Thompson while PitchingBot thinks his stuff is about average, although it also thinks that both his curveball and his cutter are plus pitches.

Keep in mind here that we’re dealing with a small sample size and that affects stuff models. I will add that Stuff+ didn’t give Thompson a single above average pitch last year, although Thompson’s cutter and curveball graded out in the 90s.

Either way, though, Stuff+ and PitchingBot give us two different pictures of Thompson. That means we need to do some investigating of our own.

Let’s start with an important question. Does Thompson have three viable pitches? I would bet against that. His fastball isn’t overly impressive, sitting 94.5 mph with only 15 inches of induced vertical break. The pitch does play up a bit because Thompson gets so much extension (6.9 feet) that it actually has a perceived velocity of 95.9 mph. That’s not enough velocity to make me not care about the shape but it is enough to make the below average shape matter a bit less.

The problem is that Thompson will likely lose some velocity when he’s pitching out of the rotation, which will make his fastball more hittable.

The next pitch in Thompson’s arsenal is a slow, big breaking curveball with a ton of drop. In fact, it has an additional 7.8 inches of drop when compared to the average 76 mph curveball. It does have below average sweep but that just makes it more of a 12-6 curveball which isn’t necessarily a bad thing.I actually like the curveball and I think that Stuff+ is probably too low on it.

That leaves us with an evaluation of the cutter. This is really the important pitch for Thompson because, if he can throw it effectively, it gives him a third pitch but it also gives him a pitch that he can use to reach the glove side as he has heavily targeted the arm side with both his fastball and his curveball this year.

At 86.6 mph, the pitch splits the velocity difference between the fastball the curveball but it doesn’t have too impressive of a movement profile. The offering has slightly below average drop and just 1.5 inches of cut, which is 54% below average. That just doesn’t seem like enough movement on a pitch that’s “only” 86.6 mph. And, in particular, it doesn’t seem like enough cut.

The problem is that the pitch doesn’t have a ton of spin (1969 rpms) and then pairs that with a low rate of active spin (24%) which is really the limiting factor in terms of movement. It’s been an effective pitch this year but I’m not confident that the results will hold.

I also wonder why Thompson throws a cutter, or at least a cutter/slider hybrid, instead of a true slider. The lefty gets a ton of spin on his curveball and generally (but not always) pitchers that can generate a lot of spin on one breaking ball can also throw another breaking ball with a lot of spin. I would be curious to see how effectively Thompson could throw a slider but I’m content to see what happens with his cutter first.

The final pitch in Thompson’s arsenal is a changeup but I’m not even going to discuss it beyond this sentence because I really don’t like it and he would probably be better off not throwing it all.

So where does that leave us? With a solid curveball, a fastball that likely won’t play well with the lower velocity that comes from pitching deeper into games, and a cutter that doesn’t have a ton of movement.

I’m not in love with that arsenal.

Now, of course, I could be totally wrong and Thompson could maximize his opportunity in the rotation and his cutter and fastball could play better than I expect them to. That’s really why I’m excited to see Thompson in the rotation.

If he flops, it’s not a big deal. He’ll just go back to the bullpen next year, which is where I think he belongs anyways. But if he’s successful, he gives the Cardinals more starting pitching depth next year and that’s always a valuable thing.

If you asked me right now where I prefer Thompson, I would say in the bullpen but I’m excited for my hypothesis to get put to the test. Hopefully my answer is different in a few months, and, if it is, it will be because Thompson’s cutter is more effective than I’m expecting it to be.

Other pitchers will get a chance

The Steven Matz injury really affects Zack Thompson the most but it does have somewhat of an effect on Matthew Liberatore too, if only to further entrench him in the rotation.

With Matz out of the rotation, Wainwright struggling, and Thompson still an unknown, Liberatore is suddenly not the weakest link. Not that results really matter all too much at this point anyways, but he really should take the ball every fifth day through the end of the season, which offers him a great chance to build on his fantastic last outing and keep tweaking his arsenal to find the right mix to give him success.

Barring some extremely dominant pitching and noticeable changes to Liberatore’s profile, the Cardinals shouldn’t be penciling him into the 2024 rotation but this does give him a chance to prove that he can be relied on if needed next year.

Beyond Liberatore, the Cardinals could turn to more young players to fill any other gaps in the rotation. That means Michael McGreevy, Drew Rom, Gordon Graceffo, or Connor Thomas could all get a chance later in the season. If the Cardinals don’t want to turn to one of those arms, then Jake Woodford could also join the rotation when he finishes his rehab stint.

Either way, there is a nice little group of pitchers getting extra chances as the season winds down.


The Steven Matz injury news is tough but it doesn’t have any look term effects. Instead it creates opportunity for some young pitchers on the staff to prove that they can pitch effectively in a starter’s role.

I’m not too confident that Zack Thompson can do that but I am intrigued to see what he does with the opportunity. At the very least, it gives us something interesting to watch in the coming weeks despite the Cardinals’ record.

Thanks for reading, VEB. Have a fantastic Tuesday.