Is a Yainer Diaz Breakout Coming?

Catcher is a thankless job. If you do it successfully, perfectly even, that means that you’re letting highly paid professionals whip projectiles at you as hard as they can hundreds of times a day. Sometimes, other highly paid professionals will divert these projectiles toward you at the last second, or inadvertently hit you with the giant wooden sticks they’re carrying. You have to dive around and flail your limbs, because the only thing worse than getting hit by one of these balls is not getting hit by them; letting them fly by hurts your team. People try to steal from you constantly, so while you’re trying to catch a rock-hard 100-mph pellet, you also have to scan your peripherals. And if all of that isn’t bad enough, here’s the worst part: Sometimes your manager chooses to start Martín Maldonado ahead of you.

I kid, of course, but I’m writing about Yainer Diaz today, and his gradual phase-in to the major leagues is a key part of his major league career so far. Diaz burst onto the big league scene in 2023 with the kind of approach that makes hitting instructors wince, then shrug their shoulders and nod. He swung early and often, took big hacks, and generally acted like he was allergic to taking walks or hitting singles. It worked. He clobbered 23 homers in only 377 plate appearances, spraying loud contact to all fields. He played better-than-expected defense, too, belying his early scouting reports.

Despite that excellent rookie season, Diaz couldn’t displace an aging Maldonado as the team’s primary catcher; he took some reps at DH, but lost those as well when the team got healthy. By the playoffs, he was an afterthought, a pure backup catcher. But when Maldonado (and manager Dusty Baker) departed in the offseason, Diaz ascended to a starting job. Then he struggled – through the All-Star break, he was hitting a so-so .284/.308/.409 with iffy defense. If you’re more of a WAR person, that’s 1.1 WAR, not great. Here’s one story you could tell about Yainer Diaz: a prospect who struggled to break through with regular playing time.

That’s the wrong story, though. It’s easy to get confused by arbitrary endpoints and focus too much on how someone’s year starts. But the second half of Diaz’s season? It was electric. He started hitting the ball with authority, peppered line drives, and hit .319/.346/.482 the rest of the way. That brought his seasonal line up to a 117 wRC+, and his career slash line to .291/.317/.476 (120 wRC+). That sounds a lot more like a top hitting catcher than my earlier descriptions would have you believe.

The point of splitting this up into too-small sections isn’t to try to tell you that one of these is the real Diaz. Decades of research and learning have taught us that splitting production into tiny buckets is a great way to delude yourself with noise. Instead, I’m hoping to demonstrate why you shouldn’t do that, and why some large-sample stats will paint you a clearer picture of what’s actually going on: Diaz is a special talent who has a chance to be one of the best hitters in baseball – if he can make one crucial and difficult change.

Here’s one way to excel at hitting: When pitchers leave you something in the middle of the plate, destroy it. That’s harder than it sounds, but let me put it this way. The best hitter in baseball when it came to run value accrued on pitches over the heart of the plate last year? Aaron Judge. The second best? Shohei Ohtani. Everyone gets some pitches to hit, because pitchers aren’t perfect machines. All you have to do is look at Judge to understand that you should never put something right over the middle – and more than a quarter of the pitches he saw in 2024 were right down the pipe anyway.

There are other ways to succeed at hitting, and it’s certainly possible to crush middle-middle pitches and still struggle enough elsewhere to bring down your overall line. But it’s a great first-order test, and it’s stable from one year to the next. About a quarter of the variation in year-two production over the heart of the plate can be explained by variation in year-one production, a meaningful number. Even better, there’s an extremely strong correlation (0.67) between heart run value and wOBA. In plain English, if you can hit the meatballs, you’re probably a good hitter, and if you can hit the meatballs once, you can probably hit them again.

Why bring this up? Here are last year’s five most productive hitters against anything thrown over the heart of the plate:

Hey, there’s our guy! And while he’s a lot like his counterparts in many ways – he hits for a high average, makes a ton of hard contact, and produces gaudy overall numbers as a result – there are some key differences.

For one, look at the discrepancy between hard-hit rate and barrel rate. For the uninitiated, a barrel is a hard-hit ball at a dangerous launch angle, likely to result in an extra-base hit. Most of these guys are hitting it high and watching it fly. Diaz is torching pitches, but he’s hitting a ton of grounders and not enough airborne missiles. Diaz hit 46% of his fair contact on middle-middle pitches on the ground last year; the other four averaged 30%. Oh, he’s hitting the snot out of those grounders, to be sure, but that’s mostly just wasted loud contact.

On the other hand, Diaz comes up empty far less often than his slugging counterparts. That’s how he’s at the top of this list despite less-than-perfect contact quality. A smashed grounder isn’t as good as a smashed liner – but it’s a lot better than a whiff. And this isn’t a matter of Diaz concentrating his swings on fastballs while others chase breaking stuff, or anything like that. If you ignore the 13 middle-middle splitters he saw, his highest whiff rate was actually against four-seam fastballs. Side note: This is mean stuff from George Kirby:

What gives with all the grounders? It’s a fundamental part of Diaz’s game, as best as I can tell. He’s run high groundball rates throughout his professional career, and every scouting report I’ve found mentions his inability to lift as a potential red flag. This isn’t some one-year deviation, with his 2023 home run binge the norm; he just hits a lot of stuff down or flat. In other words, whether you’re talking Yandy or Yainer, a Y. Diaz in your program comes with more rifled grounders than you can possibly imagine.

Does that sound bad? I mean for it to sound a little bad, but not disastrous. Not all grounders are created equal, and Diaz’s tend toward the good. Here’s one way of looking at it: In 2024, grounders hit zero degrees or lower produced a .181 batting average and .183 slugging percentage. Even hitting those hard didn’t help much; hard-hit grounders in that launch angle group went .279/.303. But hit the ball higher – one degree of launch angle or more, but still classified as a grounder – and the story changes. You can think of these as hot shots, rather than balls smoked straight into the ground. These? They produced a .442 batting average and .495 slug. The hard-hit ones went for a .575 batting average and .641 slugging percentage. Even if you can’t lift the ball to the outfield, avoiding straight-down contact does wonders.

So yes, Diaz hits too many grounders, but he does most of his groundball damage by hitting the ball hard and not straight down. He still has room to improve here, but despite his elevated groundball rates, he did a solid job of staying on plane with the ball instead of punching it down. Compare him to Tampa Bay’s burly grounder-producing Díaz, and you can quickly see a difference. Though they produced identical GB/FB ratios, 28% of Yainer’s grounders were hit above zero degrees, my “good grounder” cutoff. Only 18% of Yandy’s were. “Grounder” is an imprecise term, and it paints Yainer Diaz with an incorrect brush. He’s not that far away from producing line drives, and close actually does count here.

Another, cruder way of thinking about it. Sure, both of them produced a 1.95 GB/FB ratio, and yeah, that’s a ton of grounders for every fly ball. But Yandy’s average launch angle, aggregated across every baseball he put in play in 2024? Five degrees. Yainer’s is nine degrees, meaningfully higher. Nine degrees is in the 20th percentile for the major leagues in 2024; five is in the fifth percentile.

So here’s how I’m reading this heading into 2025: Yainer Diaz has a chance to be a dominant offensive player because he has a clear and actionable skill: When pitchers give him something over the middle of the plate, he tattoos it. Now, where he tattoos it? It’s not perfect. It could use some work. But it’s not as bad as it looks, either, no matter what a quick glance at his page says.

We data analysts are always falling for the hitters with great measurables but just one thing to fix. How many articles have been written about the groundball hitters who just need to elevate? How many about the bat control wizards who need to lift and pull more often? But many times, those are fruitless pursuits. We generalize too much, or breeze over how difficult the change is. But this time, maybe it’s different. Maybe Diaz’s groundball tendency is closer to cracking than you’d expect. Maybe the path forward is easier. Maybe he can ascend from good to great hitter with an extra degree or two here or there, where other seemingly similar groundball hitters never had a chance.

But, uh, about that plate discipline? Yeah, that might be harder to change. None of this will work if he keeps walking only 3.6% of the time – his career big league rate. None of it will work if he keeps chasing nearly half the pitches he sees outside the strike zone. Diaz’s bat produces thunder, and he’s oh so close to unlocking even more. If he can show just a slightly better combination of controlled aggression, get to the 10th percentile of chase rate instead of the first, he’ll see more pitches to hit and make more of his contact in places where he wants to, instead of where the pitcher prefers.

None of this is set in stone, of course. I can’t prove that any of this will happen, and I’m not even sure it will. But if you’re looking for someone who might have another gear, I present to you Yainer Diaz. He might not look like your average bopper. His stat page might not even look like it – way fewer true outcomes than expected, all that chase, all those grounders — but it’s there, or at least the possibility is there. Maybe the Astros have found their next middle-of-the-order threat. I’m certainly willing to believe.

Source