The Mariners selected Diaz's contract from Triple-A Tacoma on Wednesday.
Diaz has spent the entire 2026 season in Triple-A, where he's produced a 6.22 ERA, 1.55 WHIP and 83:42 K:BB over 111.1 innings. He's been used in both the rotation and bullpen in his limited time in the majors over the last five seasons, but the Mariners will likely plan for Diaz to provide some extra coverage in relief after Josh Simpson and Nick Davila both tossed 30-plus pitches during Tuesday's 22-0 loss to the Brewers. Simpson was optioned to clear room on the active roster for Diaz.