When asked Thursday why the Mets have confidence in a Diaz bounceback, Van Wagenen gave three reasons.
"First and foremost, he's healthy," Van Wagenen said. "Secondly, even in his difficult time last year he still struck out 99 batters over the course of the year. And then we know the work he's putting into his training, from a nutrition standpoint, strength and conditioning as well as nuts and bolts mechanics. I think once we hired Jeremy Hefner, Jeremy got to Puerto Rico, sat down with Edwin and we've got a good communication already set up."
Van Wagenen has said numerous times this offseason that the team is expecting a return to form from Diaz in 2020, citing their internal projection systems as one of the reasons why.
As he was struggling in 2019, Diaz had a 4.51 FIP/3.07 xFIP, struck out a still-elite 15.36 batters per nine innings (a better rate than his breakout campaign in 2018), and stranded runners at about the same rate he has during his career.
In December, the annual ZiPS projections from Dan Szymborski/FanGraphs suggested that 2019 was an aberration and saw a dominant season coming from Diaz, who ZiPS projected would have a 2.98 ERA (2.77 FIP) and 105 strikeouts in 66.3 innings. His home run rate in 2020 is projected to be 1.2 per 9 IP after coming in at a brutal 2.33 per 9 IP in 2019.
Regarding health being among the three reasons Van Wagenen gave for being hopeful in a Diaz bounceback, it's important to note that the bone spur condition in the closer's elbow might have played a part in how cautious the Mets were when it came to his usage early in 2019. If the elbow isn't bothering him at all, that's huge. As is the ability to harness his mechanics and mesh with Hefner.
But the biggest key for Diaz in 2020 could come down to his slider, an offering that pretty much abandoned him in 2019. That meant that hitters could often guess his fastball (the only other pitch he features) was coming or adjust to hit a usually fat slider that sat in the top of the zone way more than it did in 2018.
Some of what went on with Diaz in 2019 might have also been mental. But if he's able to find his slider again (fixed seams on the ball could go a long way toward that), it's fair to think he'll rebound in a big way in 2020.