The decline of Aledmys Diaz

With the news of the Cardinals trading Aledmys Diaz to the Blue Jays this weekend, many are left wondering “What happened?” To quickly summarize, Diaz defected from Cuba and arrived in the US in 2014. The following July, he was designated for assignment and any team could have claimed him off waivers. As we know, that didn't happen and he remained with the Cardinals.

He made his Major League debut in 2016 and had immediate success. He was named to the All-Star team and finished fifth in Rookie of the Year voting. His performance, although it was a small sample size, didn't appear to be a fluke. He seemed to have the necessary tools to produce. His BB% was approaching 10%. His K% might have been slightly elevated, but his ISO remained above .200. More specifically, he was chasing pitches only 28.2% of the time.

Flash forward a year and a better question might be “What hasn’t happened?”

The shortstop’s approach completely changed. He was no longer the poised hitter we all regarded in 2016. Instead, his BB% fell by 50% and his already elevated K% increased. What was the source of this change? Was it one bad year, or did pitchers make adjustments that Diaz will ultimately not be able to handle?

One noticeable difference between 2016 and 2017 is the amount of fastballs Diaz saw. In 2016, almost 60% of pitches thrown to the shortstop were fastballs. The following year, that number dropped to just above 50%. Furthermore, Diaz was not nearly as successful against the fastball in his second year as his average vs. the pitch dropped almost 20 points.

Upon examination of the location data, it will come as no surprise why this happened. Here is the pitching heatmap of all fastballs Diaz faced in 2016.

BaseballSavant

Now, I’m not saying any of us could hit a major league fastball, but if there was one location where we might be able to it’s here. Now, take a look at the 2017 heatmap for all fastballs.

BaseballSavant

This location is only slightly altered from the 2016 hot zone. In other words, it could not be the only reason for Diaz’s decline. Although pitchers adjusting to throw him fewer fastballs and, more specifically, fewer fastballs down the middle, part of the difference in success between these two years is on Aledmys.

The statistics show that he lost his patience. I have already mentioned how his walk rate plummeted. What is more alarming is that his swing% rose more than 15%. Now, that might not be a big deal. His 2017 statistics consist of a much smaller sample size than his 2016 numbers and it is possible that pitchers simply threw him more pitches in the strike zone, so he had to swing.

Upon further examination, this isn’t the case. Not only did he swing at more pitches overall, he also swung at 40% more pitches out of the zone. Again, part of this difference can and should be attributed to the smaller 2017 sample size, but the 40% number is hard to ignore. So, not only was he swinging at more pitches, he was chasing a significant amount of pitches out of the zone.

Consequentially, it is no surprise that his quality of contact dropped. His soft contact rate rose and his hard contact rate fell by almost 25%. Part of these fluctuations are the year to year difference in a players performance. It is probably too early to declare that Diaz will never replicate his 2016 success, but it is not premature to say, and it is assuredly well known now, that his value will likely never return to what it was a year ago.

That said, it does leave the Cardinals with a bit of a gap in the middle of their infield, hinting that this trade might be the first of a few corresponding moves. Whatever the case may be, the numbers reflect that Diaz could not make the necessary adjustments in 2017. His value dropped, and the Cardinals took a chance on J.B. Woodman.