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MLB Rookie Report: Seattle Mariners prospect Edwin Diaz

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Edwin Diaz
Edwin Diaz
Nat Thomas, Jackson Generals

Back in April, a reader asked for current thoughts about Seattle Mariners pitching prospect Edwin Diaz:

We aim to please, so here goes. First, the basic background from the 2016 Baseball Prospect Book:

Edwin Diaz, RHP, Seattle Mariners
Bats: R Throws: R HT: 6-3 WT: 175 DOB: March 22, 1994

2013: Grade C; 2014: Grade B; 2015: Grade B

Drafted in the third round in 2012 from Caguas Military Academy in Puerto Rico, Diaz is certainly Seattle’s top pitching prospect and (in my view) their top prospect overall. Do not be deceived by his 4.57 ERA in Double-A: his FIP was much better at 3.22 and there was no slippage in his components. The ERA was a fluke; he remains an elite prospect. Diaz gained velocity last year, going from 91-93 with peaks at 95 in 2014 to 92-95 with peaks at 98. His slider also took a step forward and is now a consistent plus pitch. The change-up needs more work but there’s enough there to project it as at least average. His control is decent and he’s been durable so far. Diaz needs more polish in the finer points of pitching but he is progressing well. He remains a strong Grade B prospect.


The reader is certainly correct: Diaz has been slicing through Double-A with little trouble, posting a 1.69 ERA in three starts for the Jackson Generals, allowing just eight hits in 16 innings with a 24/2 K/BB. At some point the BABIP will slide but as long as the K/BB and K/IP ratios remain strong, he'll continue to pitch well.

As noted in the pre-season comment he actually performed quite successfully in Double-A last year. He has little left to prove at that level and I'd have to think a promotion to Triple-A will occur sooner rather than later.

In my view Diaz is an elite pitching prospect and doesn't receive nearly enough national attention when top arms are discussed. Progress has been reported with his change-up and his command within the strike zone, something of a weakness last year, has clearly been sharper this season.

This Milb video is from last summer but gives the basic idea.

There's a little hook with his action about seconds 5-6 that may add some deception, though I know some people think it may stress his arm somewhat. The next video is from two weeks ago and that doesn't seem as prominent, though perhaps that's just the camera angle.

From the early results, Diaz has made enough progress this spring to project as a fine starting pitcher in the sense of having three pitches and strong command. The main question now is durability. The thin frame and the arm action may raise some doubts there, but he's athletic, does a better job repeating his mechanics than he did a couple of years ago, and hasn't had any serious injury concerns as yet.

My guess is that his injury risk is no higher than typical for his age. That still means it is pretty high, of course; he's a pitcher. But health allowing, he looks like an eventual mid-rotation guy to me.

UPDATE JUNE 5, 2016: After opening the season in the starting rotation in Double-A, Diaz was moved to the bullpen by the Mariners in early May and has continued to pitch excellently, leading to his promotion to the major leagues on June 4th. Overall he posted a 2.21 ERA in 40.2 innings this spring, with an excellent 54/7 K/BB and just 32 hits allowed.

The Mariners (and many outside scouts) have decided that they prefer Diaz in the bullpen due to the aforementioned durability concerns and projected better consistency with his secondary pitches in that role. My personal opinion is that he has a good future as a starter, but that's a minority opinion and the Mariners, who know him best, seem settled on relief. This may lower interest in Diaz for fantasy purposes unless he can get himself a closer job, which is possible eventually.

However they use him, Diaz has a fine combination of natural physical talent and enough refinement to make that talent impact a game.