Luetge was in his second stint with the Mariners this season. The 27-year-old lefty pitched just once after rejoining the club on April 25, throwing 1 1/3 scoreless innings in a 6-5 win over the Rangers last Sunday.
In three appearances this season, he's allowed one hit and two runs in 2 1/3 innings.
The Mariners had an extra reliever, having added Luetge when Erasmo Ramirez was sent down to the Minors on April 24. With several off days to work around, the club went with four starters for a stretch, but now adds Iwakuma to the mix and will need an extra starter on Wednesday with a doubleheader scheduled at Oakland.
Seattle now has two remaining left-handers in the bullpen with Charlie Furbush and Joe Beimel.
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB as well as his Mariners Musings blog. This story was not subject to the approval of Major League Baseball or its clubs.