It has been hard to buy Aledmys Diaz's breakout performance this season, mostly because how completely out of nowhere it has been. The 25-year-old never made a Baseball America Top-100 prospects list, and managed to post just a .264/.324/.421 line last season, as a 24-year-old at Double-A.
In fact, since joining the Cardinals organization in 2013, Diaz has never managed an OPS of even .800 for a full season; in his first 85 major-league games, Diaz's mark currently sits at .921. That Diaz has done all as a shortstop only makes his success more impressive, though no easier to make sense of.
Of course, when he started slowing down in May, posting a triple-slash line of just .276/.313/.429, it was easy to start writing off his hot start. But then he bounced back with an .848 OPS in June, and now ranks second in all of baseball in wOBA over the last 30 days, entering play Wednesday. At this point, we have to just accept that Diaz is an extremely good hitter. Still, it's fair to wonder just how he is doing this, and just how good he can be moving forward.
The biggest thing that sticks out with Diaz so far has been his plate discipline. Diaz didn't walk much earlier in the season, but that rate has been steadily increasing, while he has maintained his strong ability to avoid strikeouts throughout the season:
At no point this season has Diaz posted even a league-average strikeout rate (20.5 percent for non-pitchers) over a 30-game stretch, while his improved plate discipline lately has actually pushed him above the league average walk rate of 8.3 percent overall. When you dig into the PITCHf/x numbers, it's not hard to see how Diaz has managed this:
| Season | O-Swing% | Z-Swing% | Swing% | O-Contact% | Z-Contact% | Contact% | Zone% | F-Strike% | SwStr% |
| Diaz | 24.9% | 65.7% | 43.8% | 68.7% | 93.6% | 86.0% | 46.4% | 58.2% | 6.0% |
| Avg., non-pitchers | 29.5% | 66.8% | 46.4% | 63.9% | 86.4% | 78.6% | 45.2% | 59.9% | 9.9% |
Diaz has been better than the league-average hitter in every conceivable way when it comes to plate discipline; he swings at pitches outside of the strike zone less often, and makes contact on pitches both inside and outside the strike zone at a higher rate. His 6.0 percent swinging strike rate is good for 21st out of 164 qualified hitters, and unlike many hitters, he hasn't necessarily had to sacrifice power for contact so far.
Diaz sports a 33.1 percent hard-hit rate for the season, which is roughly average for the league as a whole, but which looks a lot better among the high-contact hitters; he ranks 14th in hard-hit rate out of 36 hitters with a swinging strike rate below 7.0 percent. That hard-hit rate supports a 12.4 percent HR/FB ratio, and his 39.6 percent flyball rate is high enough that his homer pace hardly looks out of place. Diaz's ability to put the ball in play and hit for power looks legitimate too. `
As unexpected as it is, nothing about Diaz's performance so far this season suggests there is regression on the way. At least through the first 85 games of his career, Diaz has checked off every box you want from a hitter: he makes consistent contact, has a mastery of the strike zone, and manages those things while still hitting for power consistently. Maybe he won't be a .900-plus OPS player for the rest of his career, but there aren't any glaring red flags for Diaz at this point.
Diaz emerged as one of the unexpected stars of Fantasy baseball in the first half of the season, and he might just be able to sustain it into the second half. For someone literally nobody was drafting before the season, that is a huge win.
