The Mariners recalled Diaz from Triple-A Tacoma prior to Sunday's 3-2 loss to the Angels.
Diaz had started in all but one of his 24 appearances for Tacoma this season, logging a 4.73 ERA and 120:54 K:BB in 118 innings. With all five of the Mariners' starting pitchers healthy at the moment and new manager Dan Wilson having yet to express a desire to move to a six-man rotation, Diaz looks like he'll serve as a long-relief option for now.