Edwin Diaz has changed

I’ve been thinking a lot about Edwin Diaz lately. I’ve been thinking a lot about Edwin Diaz because he has been incredible. As it’s just the beginning of May, I’ve tried my hardest to not overreact to any particular players’ performances, because the sample sizes are incredibly small. Because they are so very small, the statistics have not had time to become more stable. This will take time, but what we can do is speculate, and use several metrics to paint the fullest picture we possibly can at this point.

Several days ago, Diaz took the MLB lead in saves, and became the fifth fastest pitcher in MLB history to reach 200 strikeouts. In terms of relievers, Diaz is currently tied for 3rd in the MLB in WAR, and really, the only relievers who have truly been more lights out are Josh Hader and Adam Ottavino.

There have been a couple clear changes thus far with Diaz. The first is that Diaz is throwing his slider harder than ever.

Diaz mph


While his fastball has stayed around the same relative velocity, his slider has received somewhere around a 1.00 to 1.50 mph uptick in velocity. This has rendered hitters absolutely helpless, as his fastball and slider play well off of each other.

The second change is that he’s using his slider more, and thus his fastball less.

Diaz usage


This change in usage has optimized the values that Diaz’s fastball and slider provide him. Although it is widely considered a plus-plus pitch, Diaz’s heater has never before registered as such. It seems the effectiveness of his slider and his shift in usage may have helped his fastball play up.

When Diaz is locating his fastball on the edges of the strike zone, he’s near impossible to hit, and this year his fastball has been spectacular.

According to Pitch Info, his wFA/C (the number of runs above average batters were against his fastball) have been -0.29 and 0.45 in 2016 and 2017, respectively. This year, it’s 2.78. As for his wSL/C (slider value), it registered a 3.72 and 1.61 in 2016 and 2017, respectively. This year, it checks in at 2.34.

So far, his slider has been an absolute worm killer. The only batted balls in play that hitters have been able to manage against Diaz’s sliders have been grounders. Think about that: Out of Diaz’s 95 sliders this year, zero of those balls put in play have been line drives, fly balls, or pop ups. Further, 68.89% of sliders that hitters have offered at have been whiffs. That’s insane.

Along with more velocity, Diaz has kept his slider out of the zone more than ever.

Diaz out of zone

While pitching out of the zone didn’t make Diaz’s 2017 better than his 2016, hitters have been much worse at hitting balls pitched out of the zone this year in comparison to the years before.

Overall, these changes had led Diaz to see improvements almost everywhere. Hitters are:

  • Swinging at more pitches in the zone
  • Making far less contact on pitches outside the zone
  • Making less contact on pitches inside the zone
  • Swinging through a higher % of pitches

And yet, maybe the most interesting tidbit is that Diaz has increased the spin rate on his slider and fastball considerably.

Diaz spin rate

Since last year, the spin rate on Diaz’s slider and fastball have increased by about 1,000 revolutions per minute (rpm). This now puts him with the likes of Craig Kimbrel, Blake Treinen, Brandon Morrow, and Ken Giles.

Most likely due to his change in spin rate, the vertical movement of his pitches, particularly his slider, have both increased as well.

Diaz movement

Diaz’s newfound spin rate appears to be behind his added movement, and thus his early season successes.

And so, it looks as if Edwin Diaz has made adjustments. When pitchers make adjustments, hitters make adjustments back. Baseball, really, is a game of back-and-forth adjustments. Maybe Diaz loses his feel for his pitches. Maybe Diaz starts throwing his fastball too much. If hitters somehow manage to stop swinging at his pitches, maybe Diaz starts walking more hitters. Really, he already is walking a lot of hitters. Too many hitters.

It’s early, and so these early season results could be a whole lot of nothing. This is just a blip in the season, and maybe Sugar is simply throwing a few more sliders than usual. As recently as April 26th against the Indians, Diaz threw 73% fastballs. Maybe he didn’t have a feel for his slider, but it is certainly easy to envision a 24-year-old fireballer overindulging in a pitch of his that can surpass 100 mph.

It’s been just 14 games and 14.1 innings. Diaz’s BABIP is .105, and he’s stranded 91.7% of hitters on base. Clearly, this isn’t completely sustainable, and if you’ve watched some of his outings, it’s easy to imagine them ending up much worse than they did. One thing to keep an eye on is if he continues to walk a lot of hitters, which Diaz is prone to doing at times. That said, if he continues utilizing this mix of his fastball and slider, it would not be surprising to see him finish the year as a top-5 closer.


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