The former Japanese starter was given a long-relief role after struggling much of the spring. Iwakuma, who signed a one-year contract with Seattle, was hit hard in an exhibition loss in Japan on March 26 before pitching well in out of the bullpen on April 2 when the Mariners returned to Arizona for more Cactus League games.
But Iwakuma has done nothing more than warm up in the bullpen several times in Seattle's first 12 regular-season games.
"It's just one of those things," manager Eric Wedge said prior to Wednesday's game with the Indians. "It happens from time to time through the course of a season. It just happens this time it's the beginning of the season.
"You keep hanging on to your long guy for that next day, and that's what we've been doing. That's the role he's in. We definitely want to try and get him in there. Multiple times we've tried to do it, and it just didn't work out. You have to work off the baseball game. That's simply what we're doing, working off the baseball game. It'll happen, we'll get him in there, we just haven't been able to do it yet."
White Sox starter Philip Humber had been the only other remaining unused player in the Majors, but he started Monday against the Orioles.