St. Louis Cardinals Aledmys DiazZach Gifford | THE INTREPID STL

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Last season, Aledmys Diaz was the St. Louis Cardinals second-best hitter by wRC+. He displayed an incredible ability to adjust to how pitchers attacked him. He showed decent patience at the plate and great discipline. The result: an above average walk rate of 8.9%, good for fifth-best on the team.

Yet, twelve games and 49 plate appearances into 2017, Aledmys Diaz has yet to take even one walk, intentional or unintentional. He hasn’t been hit by a pitch yet, either. Thus, after having an on-base percentage 69 points better than his batting average in 2016, his OBP is exactly equal to his .245 batting average so far. So, what changed?

In short, he’s being more aggressive. Sandwiched between the “professional” approaches of Matt Carpenter and Dexter Fowler, it looks like Diaz is trying to ambush the pitcher. If Diaz was more aggressively attacking good pitches to hit, this wouldn’t be a bad thing. Unfortunately, he’s swinging more at almost everything.

Aledmys Diaz St. Louis Cardinals

Above, I’ve included selected hitting and plate discipline metrics. While we aren’t yet to a point where these are extremely reliable indicators of performance, most tend to be relatively quick to stabilize. Additionally, even if they aren’t necessarily important for evaluating Diaz going forward, we can use them to describe what he has done so far.

Within these numbers, there’s plenty of reason for concern. If there’s one positive in this, it’s that Diaz strikeout rate has improved thus far. However, given the changes in his other plate discipline numbers, I wouldn’t expect that to continue.

Both Aledmys Diaz chase rate (O-Swing%) and zone swing rate (Z-Swing%) have jumped significantly this year, with a 15% increase in O-Swing% and a 9% spike in Z-Swing%. Across the MLB, that brings him from the 37th and 44th percentile to the 96th and 84th (minimum of 300 plate appearances in 2016, 30 in 2017). He’s gone from showing above average patience to being more aggressive than almost everyone.

Swinging more often isn’t necessarily a bad thing if you’re swing at better pitches to hit. With that in mind, let’s take a look at the pitches he’s hacking at using heatmaps available through FanGraphs.

Aledmys Diaz St. Louis Cardinals

In 2016, Aledmys Diaz was more aggressive on pitches that were elevated over the inner half of the plate. He was noticeably more patient on balls at or below the knee, as well as those on the outer half. He rarely chased outside the zone, and when he did it was generally after pitches that were up or inside.

This year, while he’s been more aggressive everywhere, the relative changes on the outside half and at the knees stand out most. He’s being most aggressive on pitches in those areas where he was least aggressive last year. Balls on the outer half and low are the most difficult to drive with any power, and likely play a part in Diaz 2 mph drop in average exit velocity so far.

Additionally, at the same time as he’s swinging more, he’s making less contact. His contact rate on pitches outside the zone (O-Contact%) has dropped 9% and his contact rate on pitches in the zone (Z-Contact%) has dropped 3%. Given that he’s swinging more often and hitting the ball less, we’d expect a higher strikeout rate. In fact, his plate discipline numbers suggest a 19% expected strikeout rate based on my xStats model.

The lack of patience explains his extremely low walk rate as well. Using the same plate discipline numbers, my xStats model estimates he would have walked either zero times or once. That’s exactly what we’ve seen so far.

Maybe the explanation is that pitchers are getting ahead of Aledmys Diaz more often, and he’s being exposed because of it. Pitchers are throwing slightly more strikes to him this year by Zone%, and are getting ahead more with first pitch strikes (F-Strike%). It’s too early to know whether those trends will hold, but it’s something to watch going forward.

Whatever the reason, we’re seeing a different Aledmys Diaz this year. If he is going to be successful in this new mold, he will need to reign in his aggressiveness to focus on the pitches he has the ability to drive.