On the morning of April 28th, 2016, after finally collecting enough plate appearances to qualify for the National League leaderboard (however meaningless it may have been that early in the season), St. Louis Cardinals shortstop Aledmys Diaz found his name at the very top of the league in batting average. Through 66 plate appearances, Diaz was slashing .468/.500/.823 with nearly as many extra-base hits (13) as singles (16) and more walks (4) than strikeouts (3). While we all knew the sample size was still quite small and that regression was bound to happen, we enjoyed the present and promoted discussion on whether or not his bat was being wasted in the eight spot of a Mike Matheny-crafted lineup.
With Jhonny Peralta sidelined due to injury and the newly-signed Ruben Tejada viewed as a rather unappealing everyday option, the red-hot Diaz found his name in the starting lineup nearly every single day throughout the month of May -- accruing a total of 110 plate appearances (with one more day to go), essentially tripling his MLB career total. While his defense lagged behind, especially at such a critical position as shortstop, there were still few complaints about his regular inclusion in starting lineups given the state of his bat and frankly, the lack of a better alternative.
Throughout April, prior to acquiring enough reliable information on Diaz's hitting tendencies, opposing pitchers were choosing to attack the 25-year-old shortstop on the inner portion of the plate, with the primary focus being down and in but also not afraid to go up the ladder when necessary. And as is the case with just about any professional hitter consistently dealing with inside pitches, Diaz smacked a lot of line drives to left field (this image shows all balls in play on inside pitches) and was able to muscle a handful of pitches into the bleachers beyond the left field fence as well.
When pitchers left pitches over the middle of the plate or went away, Diaz was not afraid to let the pitch get deep in the hitting zone and shoot liners out to right. In fact, as you can see from the spray chart above, Diaz did this quite frequently, as seven of his 18 line drives from April were to the opposite field. It was his best 2013 Allen Craig impression.
Fast forward to the end of May (or, as already mentioned above, 110 plate appearances later) and you will find that Diaz has not yet hit another line drive to right field. Nope, not even one. Conveniently paired with the pitch heatmap, you will see that this the case despite the fact that pitchers have shifted their primary attacking point, from the inner half in April, to the outside portion of the plate in May.
Pitch Type (%) By Month (Via BrooksBaseball.net)
This table was included to show that Diaz's decline at the plate cannot be attributed to facing more breaking balls or offspeed pitches -- a popular belief brought up just about any time a young hitter begins to slump. In fact, Diaz has actually dealt with a slight increase in fastballs, though they have all been of the sinking variety. This makes the situation all the more interesting considering Diaz, early on in his career, has proven to be an "extraordinary fastball hitter."
In a vacuum, one would be absolutely thrilled to be presented with this overall line for a player who was designated for assignment roughly 11 months ago. However, and remember, this is beyond just the declining numbers in May, Diaz's batted ball profile is understandably sobering. His book has grown immensely, and it will only become more thorough for opponents as the season progresses. Admittedly, I normally would not put much stock in differences associated with month-to-month batted ball profiles due to a small sample size and the fact that they are not necessarily predictive of future results, but at the same time, I could not ignore such a drastic shift. Going 110 plate appearances without a single opposite field line drive will garner attention.
Fatigue is almost certainly an underlying factor in Diaz's decline in hitting performance. He likely has not played in this many games this early in a season in his entire professional baseball career. Fortunately, help is on the way as Peralta appears close to graduating from his rehab stint in the minor leagues, despite a recent box-cutting incident. That being said, the issue Diaz is currently dealing with goes beyond fatigue.
Pitchers have officially adjusted their approach to Diaz, so it is now time for him to return the favor. He is pulling too many outside pitches into the ground. He needs to return to driving these pitches the other way. A good start would be seeing middle pitches slightly longer before depositing them to right, instead of lunging and ultimately rolling over on them to short.
Oh, and taking a walk every once in a while could help, too.