The team isn't making any estimates on his return, but if Iwakuma can't do any throwing now until around May 22, he's likely looking at late June before he'd be ready to rejoin the Mariners at the earliest.
Iwakuma has been one of the AL's top right-handed starters the past three seasons, posting a 2.97 ERA in 77 starts from 2012-14. But he struggled in his first three outings this year (0-1, 6.61 ERA) and then was placed on the DL after an MRI test showed inflammation in the lat muscle.
He began playing catch at Safeco Field while the Mariners were on the road last week, but that didn't progress as well as expected.
"His rehab has not gone as well as we thought it would have gone," McClendon said. "He's still experiencing some stiffness, so we'll probably push him back 10 days or so. Anytime you have any kind of lat strain, you have to be careful with it. It just makes sense to back him off."
Iwakuma said he had a similar lat injury when he was in Japan, but feels this one is less serious. He's still able to do conditioning exercises and shoulder work, but can't throw.
"It is frustrating because I was hoping I'd be pain free or feel nothing by the time I was scheduled to throw," he said. "It's not feeling worse. It's better than where it was, but I feel tightness. Hopefully, next time I throw I won't feel anything."
Roenis Elias, a 26-year-old left-hander, has made three starts in Iwakuma's place and is 0-1 with a 3.86 ERA. He'll get the ball again Thursday in the opening game of a four-game series against the Red Sox at Safeco Field.
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB, read his Mariners Musings blog, and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.