High three-quarters arm slot; works exclusively from the stretch; compact arm action; max effort delivery; unimpeded arm creates violent recoil; 80-grade arm strength; uptempo pitcher; stays on top of the ball well; thick body; will need to be maintained; physically maxed out.
Four-seam fastball with 80-grade velocity; lowest fastball registered in either look was 97 mph; lived between 97-99; have seen 100 in the past; fastball effectiveness is present; doesn't nibble; solid-average command; spotted 99 on the outside black in second look; fastball doesn't straighten out; shows some arm side run at 99 mph; elite offering.
Slider comes in anywhere from 87-90 mph; consistent two-plane break; tight vertical spin; hard, diving action; late biting vertical action away from right-handed batters; essentially a 90 mph curveball; swing-and-miss offering; glove side control; will move arm slot higher at times; can flash 70-grade.
Changeup comes in anywhere from 88-90 mph; offering can fall of the table; legit solid-average offering; consistent arm speed; plays well off of fastball; 10 mph difference from the fastball; but still third best offering; keeps it where it won't be barreled; spots it down or down and away to lefties;
I have seen Diaz twice this season, both in the past 10 days, for a combined two innings of work. He's looked like a big league reliever in both looks, completely overwhelming his opposition with a three-pitch arsenal. In my second look, this was the sequence to one right-handed batter: 99 mph fastball with run; 87 mph slider with two-plane break; 99 mph fastball with run on the outside black for strike three.
The success is coming from his fastball, a true 80-grade offering that plays to its fullest potential, and recently discovered ability to throw strikes. Diaz established the fastball in both outings, throwing it for called strikes. The pitch doesn't flatten out at any velocity, including 99 mph, which is rarity. Establishing the fastball allowed his two other offerings to play up. The slider became a swing-and-miss offering in, as well as outside, the strikezone. He will throw it glove side consistently. The changeup is most pitcher's fastball, and the bottom will fall out at times.
I would fast track Diaz to the major leagues, because he may be the best reliever in the Angels organization right now. The stuff is major league ready; there's no projection in the body or arsenal, so there's no need to waste bullets in his arm. Move him up one level at a time to see if he can keep throwing strikes with the fastball.