Aledmys Diaz Hasn't Been the Same for a While

The St. Louis Cardinals just demoted shortstop Aledmys Diaz to the minor leagues. Based on some of his advanced metrics following last year's hot start, it was only a matter of time until this move occurred.

My, oh, my, how quickly things can change in a year.

In just about any sport, we can dig a quality narrative when it comes from an unlikely source, and St. Louis Cardinals shortstop Aledmys Diaz is a perfect example. Signed as an amateur free agent out of Cuba, the Cardinals actually designated him for assignment in July of 2015 to make room on their 40-man roster despite their financial investment in him. Every team in baseball had a chance to snatch him up via waivers, but nobody took a chance, and he stayed in the St. Louis organization.

He wasn't even expected to be a factor in the big leagues in 2016. But a thumb injury to Jhonny Peralta opened up a roster spot for him in April, and he took full advantage.

Diaz was one of the game's biggest surprises through his first 75 plate appearances, as he slashed .423/.453/.732 en route to instantly becoming a force in manager Mike Matheny's lineup. He was so dominant that his 215 wRC+ and .494 wOBA led the league through the season's first month.

The now-26-year-old appeared to be a crucial part of the Cardinals' future. He still may be, but as we fast forward to present day, he's back in the minor leagues.

The Hot Streak -- and Everything Else

The first month of Diaz's big league career truly couldn't have gone much better. Every ballplayer wants to experience a start that great, but it also became difficult to figure out what the "normal" was going to be for him moving forward.

His season-long numbers -- which included a .300/.369/.510 triple slash with a 130 wRC+ and a .377 wOBA -- were terrific, but they were also propped up by that scorching-hot April. The below table shows how his OPS, Isolated Power (ISO), wRC+, and wOBA changed when we separate a very successful first month from the rest of his season.

April 75 1.186 .310 .494 215
May Through October 385 .815 .189 .345 115

Clearly, Diaz looked much more human the remainder of his rookie campaign, but it's not as if he was bad by any stretch -- especially when we're talking about offensive production coming from a shortstop. Slashing .273/.352/.462 is more than respectable given the sample size, and it wasn't outrageous that the Cardinals were planning on running him out on an every-day basis again this season.

But then, the bottom fell out.

He's hit a paltry .260/.293/.396 through 288 plate appearances this year with greatly diminished power (a .136 ISO) and overall production (79 wRC+ and .294 wOBA). Add in the fact that he also wasn't an asset in the field (-9 Defensive Runs Saved in 589 innings), and this demotion shouldn't at all be shocking.

Changes in His Batted-Ball Profile

Were there any warning signs that could've told us that an offensive slump such as the one he's experienced thus far in 2017 was coming?

You bet.

Check out the below table, which shows how parts of his batted-ball profile changed between that hot April, the rest of his 2016 season, and what he's done so far in 2017. His line-drive rate (LD%), ground-ball rate (GB%), fly-ball rate (FB%), pull rate (Pull%), soft-hit rate (Soft%), and hard-hit rate (Hard%) are all represented.

Time Period LD% GB% FB% Pull% Soft% Hard%
April, 2016 22.4% 46.3% 31.3% 52.2% 11.9% 41.8%
May Through October, 2016 13.9% 45.4% 40.7% 44.9% 20.7% 29.1%
2017 17.0% 45.0% 38.0% 50.0% 23.9% 23.5%

The most concerning area here is easily his quality of contact, which has taken a nosedive since last April. And when we combine that with fewer line drives and more fly balls, it's a recipe for a deep slump.

So, despite still performing adequately at the plate from May 1st through the end of last season, he seems rather fortunate to put up the numbers he did. Plus, the .289 BABIP he posted during this time doesn't seem to be a product of tough luck -- as that number would normally suggest to the naked eye.

Diaz's BABIP so far this year is down to .283, and since his quality of contact has gotten even worse, it was hard to see any positive regression unless something changes.

Deteriorating Plate Discipline

As you might imagine, the concerning trends in the young shortstop's batted-ball profile can also be connected to his plate discipline. These numbers didn't change a whole lot throughout the 2016 season, but his approach in 2017 hasn't been remotely close to what he displayed as a rookie.

The below table uses the same time periods from above (April of 2016, May through October of 2016, and the 2017 season) to compare his chase rate (O-Swing%), swings within the strike zone (Z-Swing%), the corresponding contact rates, and his swinging-strike rate (SwStr%).

Time Period O-Swing% Z-Swing% O-Contact% Z-Contact% SwStr%
April, 2016 26.7% 68.2% 65.7% 90.2% 7.9%
May Through October, 2016 28.5% 66.0% 69.8% 91.1% 7.3%
2017 38.5% 69.0% 65.7% 89.1% 10.6%

Having one of the 15 highest chase rates among qualified hitters this season is a catalyst toward the nosedive he's experienced in his hard-hit rate, along with his walk rate going from 8.9% last season to the current 4.5% mark. Diaz has also had to deal with opposing pitchers attacking him in a different way.

After seeing fastballs 58.0% of the time in 2016, that number has gone down to 51.3% this season, along with a notable increase in curveballs (8.3% to 11.1%). It also doesn't help that he just hasn't been performing nearly as well against either offering.

Diaz has seen four-seam fastballs the most since making his big league debut. After posting a 146 wRC+ and .379 wOBA against that pitch in 2016, those numbers have decreased drastically to a 66 wRC+ and .258 wOBA. The drop in productivity has been even more significant against curveballs, as he went from a 220 wRC+ and .489 wOBA last year to a 48 wRC+ and .229 wOBA this season.

So, there are definitely some things he'll be working on during his time in Triple-A. But if he can show progress in a short amount of time, it wouldn't be shocking to see him back in the big leagues rather quickly -- just ask Randal Grichuk, who the Cards sent down and recalled in a short amount of time.

It appears that the league has made the adjustments necessary to successfully attack Diaz at the plate. Now it's his turn to adjust to those adjustments so he can get back to being the positive contributor the Cardinals need him to be.