Edwin Diaz’s Running Fastball

Edwin Diaz is having an absolutely miserable season. A year after posting a 1.96 ERA (208 ERA+), he currently holds an ERA of 5.32 (78 ERA+). He has already given up 10 homers in 44.0 innings, whereas last year he gave up just 5 in 73.1 frames. Some of his stats, such as his strikeout rate of 14.5% and walk rate of 3.3%, while less impressive than last year, are sitting at about his career averages. Mets mananger Mickey Callaway has often cited his mechanics as the main problem, and that when he throws more “sidearm,” it is a recipe for disaster. To get a visual of this difference, notice the release point on the following two pitches:

Notice how Diaz’s arm is much flatter in the first picture. The release point is a bit farther from his body and significantly lower. Pitches released in that way have too often resulted in a running fastball:

From this angle, however, it is difficult to see the exact difference in release point because one may be farther forward than the other. Consider the following table that tells a more detailed story:

Edwin Diaz Four-Seam Fastball Metrics
Year Velocity Spin Rate Horizontal Movement Vertical Movement Release Extension Horizontal Release Position Vertical Release Position
2016 97.0 2263 -0.944 1.48 6.61 -2.28 5.47
2017 97.4 2288 -0.930 1.34 6.64 -2.44 5.29
2018 97.3 2341 -0.888 1.30 6.63 -2.70 5.18
2019 97.3 2381 -1.04 1.20 6.70 -2.86 5.13

First, note that the velocity and spin rate in 2019 are similar to 2018, although the spin rate has steadily increased over time. The horizontal movement is at a career high this year, which is consistent with the perception of more arm-side run on the pitch. Combining the last three variables, Diaz is releasing the ball further out in front, farther from his body, and lower than he ever has. Interestingly, however, this was also the case last year with respect to the horizontal and vertical release positions. This might explain how he fell into the habit, as it worked well for him last year, but in 2019, it seems he has taken it too far. It may also be that with the increased extension, he is lunging forward too much and the combination is throwing him off balance.

Next, take a look at how each release metric has behaved.

Release Extension

The density plot above reveals that in 2019, Diaz has been pretty hit or miss with his release extension. There are two peaks in the graph, one that is consistent with previous years, and one to the right. The rough middle of the two peaks is 6.7, so using that number as a cut point, consider Diaz’s stats on pitches released beyond 6.7 feet (“extreme” group) and pitches released before 6.7 (“normal” group):

Edwin Diaz Fastball Metrics by Release Extension Type
Year Type Pitches Horizontal Movement Vertical Movement Release Extension Horizontal Release Position Vertical Release Position xwOBA on Balls in Play
2016 normal 99 -0.932 1.47 6.49 -2.25 5.51 .398
2016 extreme 52 -0.967 1.51 6.85 -2.34 5.40 .759
2017 normal 487 -0.929 1.33 6.49 -2.45 5.32 .319
2017 extreme 285 -0.933 1.36 6.88 -2.41 5.25 .375
2018 normal 474 -0.890 1.30 6.50 -2.73 5.19 .345
2018 extreme 253 -0.884 1.29 6.86 -2.65 5.16 .322
2019 normal 293 -1.02 1.21 6.53 -2.87 5.16 .366
2019 extreme 269 -1.05 1.18 6.88 -2.84 5.10 .420

Batters are making much better contact against pitches in the “extreme” category this year, and although this was also true in ’16 and ’17, he actually had more success with that release position in 2018. This is another indication that he may have found success with the long extension and fallen into a habit last year. Also, notice that the horizontal movement has increased in both categories in 2019, so it is not the case that the different arm angle is leading to the extra tail on the pitch.

Diaz has also thrown more balls out of the strike zone with the farther release in 2019:

Again, it was not much of an issue in 2018, but this year many of the “extreme” pitches are missing up and away.

Horizontal Release Position

The horizontal release position has been consistently moving farther away every year, to the point where the peak of his current distribution is at a position that was not even on the map until last year. Using the rough center of the distribution at -2.875 as the cutoff, consider the stats where “extreme” is now defined as farther than -2.875 feet. (No pitches fit that criteria before 2018, so only the last two years’ stats are displayed.)

Edwin Diaz Fastball Metrics by Horizontal Release Position Type
Year Type Pitches Horizontal Movement Vertical Movement Release Extension Horizontal Release Position Vertical Release Position xwOBA on Balls in Play
2018 normal 611 -0.873 1.29 6.64 -2.65 5.18 .310
2018 extreme 116 -0.967 1.31 6.53 -2.97 5.18 .508
2019 normal 291 -1.04 1.23 6.72 -2.72 5.16 .314
2019 extreme 271 -1.04 1.16 6.68 -3.01 5.11 .452

The difference in performance in this case is stark, and it has been the same for two years. When Diaz extends his arm too much, his fastballs get absolutely crushed. As before, the horizontal movement is consistent between groups.

The pitch distribution reveals he has also struggled to throw strikes with the extended release point in 2019.

It makes sense that the wide release would tend to result in arm-side pitches, but notice a key difference in 2019. First, he has thrown a lot more pitches from the extended position, and while in both years he got hit hard on them, it has been exacerbated this year by the pure number of those pitches. Second, in 2018 the pitches tended to be closer to the corner of the plate. Although many of the strikes got hit, he was not hurting himself with balls out of the zone. In 2019, in addition to the damage being done on balls in play, he is throwing a ton of pitches outside the zone for balls, most of which match the location of the “tailing” fastball arm side and up.

Vertical Release Position

Diaz’s release point has gotten lower over time, although the jump from 2018 to 2019 was not too large. The cut point for the vertical release position is 5.1, which is the approximate peak of the 2019 distribution, and anything below 5.1 feet is the “extreme” group. Again, since not many pitches were in that group before 2018, only the last two years are presented.

Edwin Diaz Fastball Metrics by Vertical Release Position Type
Year Type Pitches Horizontal Movement Vertical Movement Release Extension Horizontal Release Position Vertical Release Position xwOBA on Balls in Play
2018 normal 514 -0.877 1.31 6.60 -2.73 5.24 .371
2018 extreme 213 -0.916 1.26 6.70 -2.65 5.03 .280
2019 normal 323 -1.03 1.23 6.68 -2.83 5.21 .434
2019 extreme 239 -1.05 1.16 6.73 -2.89 5.03 .344

In this case, the “extreme” group has actually seen more success in both years. This seems to contradict the idea that the sidearm-like delivery is the problem. The pitch distribution reveals that it is not quite that simple:

The lower release point has seen many more pitches out of the zone than last year. However, with the effectiveness on balls in play, it might be worth keeping the lower delivery and learning how to throw strikes.

Key Takeaways:

  • Diaz’s mechanics are likely difficult to correct, as they have consistently changed from year to year.
  • The most drastic dip in performance has occurred when his horizontal release position is farther from his body, whereas the vertical release point seems less important.
  • None of the differences in release position change the velocity, spin rate, or movement of his fastball.

These mechanical issues suggest, as the Mets have indicated, that Diaz can find his form again. Only time will tell if he will be able to make the necessary adjustments to return to his elite status.

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K/9 and BB/9 instead of %.