What’s wrong with Aledmys Diaz?

It’s no secret that Aledmys Diaz has struggled this season. After putting together a dream 2016 season as a rookie, the Cuban-born shortstop has fallen short of expectations in 2017.

Diaz batted .300/.369/.510 with a .370 wOBA and 132 wRC+ in 2016. He struck out just 13 percent of the time, and had a sustainable .313 BABIP. His rookie season was so impressive that Steamer forecasted a .339 wOBA and 111 wRC+ for Diaz this year. The FanGraphs readers predicted a .348 wOBA. For a 26-year-old shortstop with just 460 big-league plate appearances, those were lofty projections.

But unfortunately for Diaz and the Cardinals, things haven’t gone as well this year. He’s batting just .258/.292/.400 with a .294 wOBA and 79 wRC+.

What’s behind his struggles? There are a number of problematic trends. First, his plate discipline:

Diaz is swinging at pitches out of the strike zone way more often this year than he did last year. His overall swing rate is up, and his contact rate is down, meaning his swinging-strike rate has taken a significant step in the wrong direction.

What does Statcast have to say?

At least his average launch angle is the same? Everything else has taken a dramatic turn for the worse. Compared to last season, Diaz’s average exit velocity is down 3.7 mph, his xwOBA is down 52 points, and perhaps most importantly, his percentage of balls in play hit at 95+ mph is down more than 15 percentage points.

There are diminished returns associated with exit velocities above 95 mph. In other words, hitting the ball hard is hitting the ball hard. An exit velocity over 95 mph is relatively good no matter what, and there’s little difference between hitting the ball 105 mph and 115 mph.

Diaz’s 39.2% rate of hitting the ball 95+ mph last season was not necessarily among the very best in baseball, but it was comfortably in the top half.

This year, however, Diaz’s 24.1% rate of hitting the ball 95+ mph is well below league average and in the same range as notable defense-first players like Gorkys Hernandez and Nick Ahmed.

Given Diaz’s clear struggles across the board, it may come in handy to get more specific. Are pitchers approaching him differently this season than they did last year?

Yes. They’re throwing fewer fastballs, and more sliders and curves. Why are they doing so? Here’s how Diaz has fared against specific pitch types. The following metric shows the run value produced per 100 pitches against the given pitch type:

Last year, Diaz did his best work against sliders and curves (discounting splitters, because he saw them just 0.6% of the time). The only pitch Diaz didn’t fare well against was cutters.

This year, he’s done well against cutters and changeups, but that’s it. He’s been worst against fastballs (again discounting splitters because he’s seen so few), and he’s also done poorly against sliders and curves, the pitches he’s seeing more often this year than last year.

Statcast provides a more detailed analysis of how he’s fared against specific pitch types:

Against fastballs, Diaz just isn’t hitting the ball as hard this year as he did last year. His average launch angle against them is pretty much the same, but there just isn’t as much velocity on his balls in play.

It also looks like Diaz got extremely lucky last year against sliders and curves. His xwOBA was 118 points worse than his actual wOBA against those pitches in 2016.

His wOBA against sliders and curves this year is just .290, much closer to his .245 xwOBA. Interestingly, his launch angle against them last year was just 5.1 degrees, and this year it’s all the way up to 11.3 degrees. As is the case with fastballs, Diaz just isn’t hitting sliders and curves as hard this year as he did last year (although the decline in average exit velocity is less pronounced).

In summation, things don’t look great for Aledmys Diaz.

FanGraphs’ Depth Charts projection system, which averages Steamer and ZiPS with playing time adjustments made by FanGraphs staff, forecasts a mediocre .325 wOBA and 102 wRC+ for Diaz for the rest of the season. If he continues to play poorly, those numbers will dip even more than they already have.

Diaz’s ground ball and fly ball rates haven’t changed much from last year to this year, but he is pulling the ball slightly more often and going to the opposite field slightly less often. His main problems are clearly his plate discipline and exit velocities.

Swinging at pitches out the strike zone at an above-average rate is not a recipe for success. He’s made contact on 67 percent of swings at pitches out of the zone this year as well, which is about four percentage points higher than the league average. Making above-average contact on pitches out of the zone could at least partially explain the dip in exit velocity across the board.

Diaz’s rate rate of balls hit 95+ mph falling so dramatically is also a concern. If he wants to produce anything like he did last year, he needs to hit the ball hard more consistently.

Of course, it also looks like some of the numbers from his excellent rookie season were inflated by unsustainable results against sliders and curves, so perhaps there was an unreasonably high expectation of success coming into 2017. After all, his xwOBA last season was just .322, compared to his actual wOBA of .370.

Diaz’s struggles can perhaps be explained as the league adjusting to a young hitter. If that’s the case, Diaz just needs to adjust back. Depth Charts’ projection of near-league-average production seems to suggest he’s capable of doing so. However, this miserable three-month stretch to open the season has rather drastically driven down his expected numbers, and the evidence presented today supports the diminished outlook.

All stats current through June 22.

Ben Kaspick is a contributor for Beyond the Box Score and RotoGraphs, and the founder of CoveCast, a saber-slanted S.F. Giants podcast. You can follow him on Twitter at @benkaspickor @Cove_Cast.