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STATS Advanced Metrics: Trade for Edwin Diaz Gives New York Mets Bullpen Much-Needed Relief

By: Stats Perform

The New York Mets kicked off the month of December with an intriguing blockbuster trade, making a deal with the rebuilding Seattle Mariners. Robinson Canó, Edwin Díaz, and a stack of $20 million were sent off to New York. In return, the Mariners acquired Jay Bruce, Anthony Swarzak, Gerson Bautista, and prospects Jarred Kelenic and Justin Dunn. Quite the haul for both teams, in fact.

Seattle bagged a couple quality prospects to stock its farm system and the Mets landed some money to offset the massive contract they’re absorbing with Canó. But the headliner of the deal was Díaz. The young Puerto Rican fireballer burst onto the scene in 2018, and added the following accolades to his résumé: All-Star, AL Reliever of the Year, and MLB Saves Leader (57).

With the acquisition of Diaz, who posted a 1.96 ERA last season, the Mets can definitively say they have improved their bullpen (the group put up an anemic 4.96 ERA last season). Whether or not that leads to more wins than no decisions for NL Cy Young Award winner Jacob deGrom is an entirely different conversation.

So what makes the young Díaz such a special pitcher? He puts food on his table by way of a devastating one-two combo: a white-hot four-seam fastball and a beautiful wipeout slider. We will analyze and measure the success of both pitches via the following metrics developed by STATS:

  • xRVAA: How many runs a pitcher is better than the average
  • xRVAA-: The expected Run Value above average per plate appearance
  • Whiff+: Ability to generate swings and misses
  • Discipline-: Ability to induce poor decision making from the batter

With this knowledge at our disposal, we will be able to see why Mets fans have something to be excited about going into Spring Training.

Four-Seam Fastball

Díaz operates at a lower arm slot, allowing him to get legitimate life to his fastball, which in turn led to solid marks across the board in STATS metrics. Per the xRVAA numbers, he prevented about eight more runs relative to the average pitcher with his fastball. This number was 24th overall among hurlers, despite the shorter workload of a late-inning reliever. In terms of xRVAA-, he only gave up 60 percent of the runs an average pitcher would have with a four-seam fastball.

His ability to get swings and misses from hitters was also well above average. He generated 26 percent more per pitch than the average pitcher with his heat-seeker.

However, he was more in the middle of the pack when it came to our Discipline- metric, meaning he wasn’t quite as effective in baiting hitters into bad swings. His plate discipline against was eight percent worse than the average pitcher when throwing heaters.

In all, his fastball graded out to have a sWAR of 1.3, which ranked near the top of relievers in all of baseball. The highest sWAR a qualified Mets reliever had for a four-seam fastball last year came from Seth Lugo at 1.1. The addition of Díaz could allow Lugo and his deep repertoire to be used in a setup role, something he may be more suited for. Doing so could wind up taking some pressure off of the bullpen.


Díaz’s slider is truly his bread and butter. While he only threw 438 of them, he made the most of each one. In regards to xRVAA, he prevented about five more runs compared to the average pitcher. His xRVAA- showed that he gave up a wicked 64 percent of the runs an average pitcher would have, ranking amongst the highest in the league.

This is where things get really exciting. His Whiff+ had an incredible rating of 234, meaning that he generated 134 percent more swings and misses per slider than the average pitcher. New Washington Nationals starting pitcher Patrick Corbin led the entire league at 272, making Díaz’s slider look all the more impressive.

His Discipline- rating was also sparkling. Díaz’s mark came out at 48 (fourth overall), meaning that he was 52 percent better than the average pitcher at inducing poor decision making from the hitter. Once again, he rated slightly behind Corbin’s slider (47).

Finally, his sweeper finished the year with a 0.7 sWAR, which may not seem like much at first glance. However, his slider did finish 40th overall in the sWAR rankings and 70th in usage. So if extrapolated over a longer period of time, it certainly looks prosperous. Just for perspective, the only rostered Mets pitchers that have a higher sWAR for their sliders are deGrom and Noah Syndergaard.