Yet, despite avoiding injury for the first full season of his career, in 2023 Tatis had his least productive year at the plate. His 113 wRC+ was 41 points below the mark he had recorded over his first three big league seasons. He also set career lows in average (.257), on-base percentage (.322), slugging (.449), ISO (.191) and wOBA (.332). From a data perspective, his quality of contact took a significant hit, though that isn’t all that surprising. Even after athletes return to the field, it takes time for them to regain their explosiveness following serious injuries and surgeries. Ronald Acuña Jr. is a perfect example of that. He tore his ACL in July 2021, underwent season-ending surgery, and missed Atlanta’s first 19 games of 2022. Like Tatis last year, Acuña was mostly healthy for the rest of the season but did not perform up to his standards. Then, of course, last year he won the NL MVP and became the first player ever to hit 40 home runs and steal 70 bases in a season.

Tatis will look to take a similar path, but in order to do so, he’ll have to figure out and address the root causes (mechanics, swing decisions, etc.) of this big drop off. Back in September, Ben Clemens investigated how spray angle on fly balls impacts some of the hardest hitters in the game, Tatis being one of them. One of the key conclusions of Ben’s research is that hitters who pull their fly balls at an extreme rate, such as Isaac Paredes, don’t do more with those batted balls; they just hit them much more frequently, which allows them to outproduce others on fly balls, despite not having the eye-popping power that we’d assume would be the main causal variable.

This is notable for Tatis because, over his first three seasons, he hit the ball with enough power to do damage on fly balls no matter the spray angle. That was not the case last year.

From 2019 through 2021, Tatis’ wOBA on fly balls slightly outpaced his xwOBA, while last year, he greatly underperformed his expected numbers. Much of that can be explained by his pull rate dropping from 30.5% over his first three seasons to 21.8% in 2023. However, that’s not the only variable at play here, because even when he hit straightaway fly balls in those first three years, he had a .764 wOBA. That’s well below his .894 xwOBA, but it was still the third-highest mark among all batters from 2019–21 (min. 150 fly balls). Last season, though, Tatis finished with a .291 wOBA on straightaway fly balls, significantly lower than his .653 xwOBA. Crushing balls to the deepest parts of the park was once a Tatis superpower; in 2023, it was his kryptonite.

To better understand how this happened, let’s look at how Tatis fared in different areas of the strike zone. By breaking down his performance in different zones, we’ll get a better idea of any holes that may have developed in his swing. For all his woes last year, Tatis continued to rake against left-handed pitching (152 wRC+), so I’m going to focus on his splits vs. righties, against whom he had a career low 101 wRC+. The table below shows how Tatis performed against pitches from righties in each third of the zone, first in 2021 and then in 2023.

There are drops across the board here, but my goodness, what the heck happened on the outer third? In 2021, Tatis ranked second in right-on-right wOBA on outer third pitches. This year, if you were a righty who could locate on the outer third, Tatis would do the work for you.

What’s the deal? Making contact wasn’t the problem, because he actually whiffed less often against the outer third last year than he did in 2021. The real issue was his quality of contact. On his 67 batted balls against pitches from righties on the outer third in 2021, his xwOBACON was .634. In 2023, it was .305. Ooof.

A change in swing path is typically to blame when a player goes from destroying the outer third to hardly covering it at all. Let’s look at the video to figure out what mechanical flaws altered his swing path. Here are some swings from 2021 against outer third pitches from righties.

Here is a standard heater away with a pretty neutral body angle (sorry Keegan Thompson):

This one is a low and away breaking ball with a pretty aggressive body adjustment to get the barrel under the ball:

And lastly, here is a high heater that needed an upright body adjustment:

Each of these three swings shows how Tatis adjusted his body in different ways to get to his barrel to outer third pitches. The swings are reciprocal, athletic, and vicious. His stability with the ground is consistent no matter the posture of his upper body. Now let’s look at three swings on similar pitches from 2023, starting with another standard middle-away heater:

Here is a swing on a low breaking ball with a body adjustment where Tatis couldn’t quite create the same angle as he did in 2021:

Then here is a can of corn fly ball to center on an up and away heater where Tatis caught it off the end of the barrel:

OK, now for some comparison. Off the rip, it’s clear that Tatis’ stride is working in a different direction. It’s more neutral now than it used to be. Instead of working from a neutral stance into a closed stride, he’s working from an open stance into a neutral stride. The starkest comparison is looking at how he handled Thompson’s heater in 2021 versus the 2023 one we saw from Cristian Javier. Against Javier, it was the exact kind of pitch you’d expect Tatis to drill into the opposite field gap, but his legs didn’t create enough space for him to get his barrel moving in the optimal direction. Instead of a laser opposite field homer, it was a measly liner to left for an easy out.

In 2021, Tatis had a more stable base, which allowed him to create a more drastic angle with his upper body against the low breaking ball. That made the difference between his line drive in the gap from 2021 and last year’s line drive to the shortstop. On both of the high pitches, he had the tall posture he needed to get on plane, but in 2023, he couldn’t get his bat on the proper horizontal angle to make flush contact, causing him to hit the ball off the end of the bat instead of the barrel.

As I always say, we’re looking for reciprocal movements. If he’s still kicking back aggressively but doesn’t have the movement beforehand to make the kick back smooth, then he’s creating asymmetrical movements. The closed stride and smooth kick back was his recipe for success in 2021. The logic here is that when he strides closed, he has a more stable connection to the ground, leading to better positions to get his barrel on plane.

He did not do that last year, and as a result, he created less space for his upper body to cover the outer third effectively, which sapped his production on fly balls. Because he was coming back from surgery, it’s possible that he wasn’t comfortable making the same movements he had in the past, though it’s hard to believe that would be the only reason for losing his mechanics. After all, his surgery was on his shoulder, and this is a lower body problem. That said, even if he knew what was wrong, his shoulder could have limited the amount of extra swings he could take to fix it during the season.

Now that he is healthy, he should be able to do the drills and cage work necessary to correct his mechanics and return to his previous rotational patterns.

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