Does Ronald Acuña Jr. Help His Teammates See More Fastballs?

As somebody who grew up playing baseball and continued to play through college, I’m familiar with all the old school adages that this game has. As I tried to develop my own abilities, I needed to figure out which were true, which were false, and which had some grey area. Swing down, for example, is not necessarily what hitters do in practice, and I had to relearn that. Then there is the “squish the bug” cue for your back foot. That actually makes most hitters get too spinny and reinforces an incorrect interaction with the ground. These are just a few hitting examples. But for this piece, I’m going to focus on stealing bases.

When an aggressive base stealer is on first, they command a level of mental real estate in a pitcher’s mind, though it’s important to note not every pitcher is the same. Some are more equipped to handle a runner than others. That could be due to a good pickoff move, quick time to the plate, or simply because a guy remains unfazed no matter the situation. This is all common knowledge. What isn’t, and what I’ve been pondering as one of the grey area tropes, is whether pitchers change their pitch mix when there is a threat to run. In general, I always thought this was true, but I never went back and looked at any data to confirm it. But we live in a baseball world with solid data accessibility, and luckily, this question can be answered thanks to Baseball Savant.

What makes this relevant is the conversation around Ronald Acuña Jr. and his potential impact on what pitches his teammates see when he is on base. To be clear, this exercise can be done for any player or across the league, but I’m using Acuña as an example because nobody has stolen more bases (or attempted more steals) than him this season. The context of the team matters a ton here, too: If Acuña creates a competitive advantage (even if it’s small) by being a threat to run, then he should absolutely exercise that.

The theory or expectation is that if a pitcher is concerned a runner might take a bag, they’ll want to give their catcher a better chance to catch that would-be base stealer, and the best pitch to do so with is a fastball. Breaking and offspeed pitches move in different directions and create more room for error on a transfer.

Let’s get to the data. I’ve chosen an arbitrary cutoff point of 100 pitches seen when Acuña was on base. That left me with three talented hitters: Ozzie Albies, Austin Riley, and Matt Olson. Those three all typically hit in the three spots after Acuña. I’ll start by showing you how pitchers attack each of them overall:

This is just to create a baseline for how each of these hitters is attacked. Albies sees the most offspeed pitches, Riley sees the most breaking balls, and Olson sits in between each of them in every category.

Next, here’s what each of these hitters see when there is only a runner on first base, not including Acuña:

Comparing to their baselines, only Albies sees an increase in fastball usage when there is a runner on first. Both Olson and Riley see slight decreases, but nothing significant. For Riley, breaking balls are the most effective way to get him out, so you might not want to stray from that plan too much. The same holds for Olson and offspeed pitches.

Let’s shift to when only Acuña is on first base with no other runners:

That’s a hefty difference compared to when other runners are on first, or in any situation for Olson and Albies. Riley’s mix is anything but crazy, looking similar to the other scenarios already presented, but he also has seen far fewer pitches than the other two with only Acuña on first. And he may not have seen a pitch group change, but the share of sinkers shoots up a ton from 20% at baseline to 27.5% with Acuña on. I can attest that four-seamers are much easier to catch and throw than running sinkers, but that doesn’t make it an advantage for Riley.

Either way, the case for Albies and Olson is undeniable: they’re seeing more fastballs compared to the other two scenarios. That’s a good advantage for both, since they’re best against that pitch group. It’d be nice if they were clearly hitting better because of the higher fastball rate, but we can’t just assume so.

Now let’s look at each player’s performance in the presented three scenarios:

If you were expecting fully conclusive numbers here, sadly, that is just not how this sport works.

Let’s go player by player. Olson has had a strong preference for Acuña on first versus any other Atlanta position player. He has also been better than his overall baseline from an expected and actual outcomes standpoint. Albies is a bit different. His wOBA with Acuña is better than his baseline, but his xwOBA is .021 ticks lower, and for some reason, he has been absurdly good with non-Acuña runners on first. Is this a second or third time through the order thing? Could it just be random noise? It’s not clear. For Riley, his wOBA is also significantly higher with Acuña than the other two scenarios, but his xwOBA stays steady across the board. I wonder if the favorable gap between wOBA and xwOBA when Acuña is on for all three has anything to do with him also taking up mental real estate in infielders’ heads, but that’s a question for a different piece.

There are plenty more ways to analyze this situation, but to be honest, I was most interested in answering the question of whether a pitch mix change exists or not. For the hitters who are up the most when he’s on base, there is no question it has. But I’ll continue to stress that this is highly context dependent. What we see for Acuña may not be the case for other players. For now, at least we have an answer: Acuña is most likely helping his teammates see more heaters. What they do with those pitches, though, is completely up to them.