The Mariners designated Diaz for assignment Saturday.
Diaz gave up one run on two hits and struck out a batter in 1.1 innings during his lone appearance with the Mariners after being called up from Triple-A Tacoma on Wednesday. He'll now lose his spot on both the active roster and 40-man roster as the Mariners make room in the rotation for Kade Anderson, who was called up from Double-A Arkansas.