The plan is for him to resume playing catch Friday.
"I'm being optimistic," Iwakuma said through translator Antony Suzuki after receiving the MRI results. "I'm being very positive with the situation. It's minor. It's not bad, as we saw on the MRI, so I want to come back soon and help the team and contribute."
Iwakuma (0-1, 6.61 ERA) had yet to pitch well this season in his three starts. On Monday, he lasted 5 1/3 innings, allowed four runs and took a no-decision in the Mariners' 7-5 loss to the Houston Astros. His numbers weren't bad, but his velocity was down and his splitter wasn't sharp. And Mariners manager Lloyd McClendon lifted him after just 71 pitches.
Iwakuma said he had a similar injury in 2006 on the first day of the season while pitching in Japan, but that it was more serious.
"That was bad," he said. "But this is just a mild strain, so this is a lot better."
His injury comes as a blow to a Mariners starting rotation that entered Saturday 13th in the American League with a 5.05 ERA. Iwakuma has been one of the club's best pitchers since coming to the Major Leagues in 2012. He made an All-Star appearance in 2013 and went 15-9 with a 3.52 ERA last season.
Asked how the loss of Iwakuma will impact the rotation, McClendon played coy.
"I don't know,' he said. "We'll see how tomorrow's starter does and I'll let you know."
That starter will be left-hander Roenis Elias, McClendon announced, though the club wasn't expected to to officially recall him until Sunday from Triple-A Tacoma, where he is 2-0 with a 8.04 ERA in three starts. Elias was in the Mariners clubhouse Saturday.
In 2014 with Seattle, the Cuban defector went 10-12 with a 3.85 ERA as a rookie.
Adam Lewis is a contributor to MLB.com This story was not subject to the approval of Major League Baseball or its clubs.