Iwakuma becomes the fourth Mariners starting pitcher to be placed on the disabled list. Yovani Gallardo is the only starting pitcher from the team's projected rotation to stay healthy this season.
The Mariners placed left-hander James Paxton on the 10-day disabled list last Friday with a grade 1 forearm strain. Left-hander Drew Smyly (flexor strain) and right-hander Felix Hernandez (shoulder) also are on the disabled list.
Iwakuma, 36, is 0-2 with a 4.35 ERA in six starts (31 innings this season). The Mariners made his disabled list move retroactive to May 4.
Right-hander Sam Gaviglio, 26, was recalled from Triple-A Tacoma to fill Iwakuma's spot on the 25-man roster. He is 2-3 with a 3.31 ERA in five minor league starts (32⅔ innings) this season.