Some days you just wanna take a Jonathan Papelbon (48 starts in the minors with a 3.05 ERA and crushing SO and hit totals) and turn him into a flame-throwing pen monster without worrying about "time to adjust to the big league game" or "can he hold up for 200 innings every year?"
I expect Diaz to be a very good bullpen arm, just like Archer would have been or Chris Sale was before he was given his release papers and turned loose on the rotation. Diaz's minor league stat-line is nearly identical to Papelbon's, and he and Archer have similar body types. I would have let him and Taijuan duke it out for the young TOR spot, but the Mariners wanted bullpen help and they didn't trust that Archer Body.
You make those calls and hope they work out. Diaz should have a lot of fun detonating 3 batters every close - I just wanted to see him do that for 180 innings instead of 70. Bring on his next appearance, please. I'll be able to enjoy that kind of "disappointment" for several years for sure.