The Mariners recalled Diaz from Triple-A Tacoma on Tuesday.
Diaz will take Gabe Speier's spot on the 26-man roster, marking Diaz's second MLB stint of the season. The 27-year-old's lone appearance with the Mariners came in June when he allowed three runs on nine hits in 5.1 innings as a starter. He's expected to serve out of the bullpen during this stint but could be called upon if the Mariners need a last-second starter. In 18 Triple-A outings (17 starts), Diaz has a 3.88 ERA with a 95:42 K:BB during 97.1 innings.