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Aledmys Diaz struggles go beyond poor plate discipline

Aledmys Diaz St. Louis Cardinals

photo by @cardinalsgifs

Aledmys Diaz is under heavy scrutiny for his performance at the plate in 2017. After compiling a 132 wRC+ in an impressive and unexpected rookie campaign, his performance has tanked to an 81 wRC+ through nearly two months this year. That drop has mostly been attributed to an overaggressive approach at the plate and poor luck on balls in play.

However, there are also indications that Aledmys Diaz has lost some strength in his swing. His average exit velocity is down nearly 4 mph, currently sitting at a below-average 85.0 mph. Additionally, his isolated slugging percentage (ISO) has dropped 71 points from .210 to .139.

Some of that is due to poor swing selection. Diaz is making contact with more pitcher’s pitches and is unable to drive them. Yet, even when he hits the pitches we want him to hit, pitches that are strikes over the inner half of the plate, he’s not doing the same damage he did last year.

Using the detailed zones available in the Baseball Savant Statcast Search, I filtered to find batted balls on pitches that were located on the inner-half and were not borderline pitches. You know, these kind of pitches…

Aledmys Diaz St. Louis Cardinals

Since the beginning of 2016, the MLB average exit velocity among right-handed hitters against these pitches is 91.1 mph. Last year, Aledmys Diaz generated an average exit velocity of 93.1 mph against 150 pitches in this zone, good for a .789 SLG and .466 wOBA.

This year, on 54 batted balls, his average exit velocity against these pitches has dropped all the way to 88.0 mph, his SLG to .571, and his wOBA to .389.

Possibly even more troubling, Diaz has experienced these drops even while maintaining a line drive rate around 25% (24% in 2016, 26% in 2017). The exit velocity problem is actually even more apparent on his liners: after averaging a 98.2 mph exit velocity on line drives in 2016, he’s down to 91.3 mph on 14 liners this year. That’s nearly a 7 mph drop. He’s fallen from the 63rd percentile among RHHs to 7th. And again, these numbers are only for line drives hit on pitches over the inner half and away from the edges of the zone. There’s no reason he shouldn’t be hitting them as hard as he did last year, unless he now lacks the bat speed to generate comparable exit velocity.

My initial thought was that Aledmys Diaz might have lost some strength after fracturing his thumb last July. Yet, while the post-injury sample size was only 14 batted balls, Diaz generated a 93.6 mph exit velocity post-injury on balls in the selected part of the strike zone, compared to a 93.0 mph average on 136 batted balls pre-injury.

That initial small sample doesn’t indicate Diaz strength was sapped. So while this year suggests otherwise, maybe we’re just seeing an anomaly that doesn’t reflect Aledmys Diaz actual talent level. After all, exit velocity reliability is strange. It is most reliable at 50 balls in play, however.

Or maybe we’re seeing what Diaz looks like after pitchers had a chance to scout him more carefully, or why 29 teams passed on him when he was DFA’ed in 2015.

The truth probably lies somewhere in between. Diaz might not be as good as he showed last year and might not be as bad as he’s shown this year. He’s posted a 104 wRC+ so far in May, so maybe that’s the best reflection of who he is as a hitter, although I have as many (perhaps more) concerns with his contact profile this month than I did in April.

Diaz showed a strong ability to make adjustments on the fly last season. If he’s going to get back to a higher level of production, he might need to start making adjustments again.

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