If there are no setbacks for the 33-year-old, that would put him on schedule to go out on a Minor League rehab assignment in late June, and possibly be in line to rejoin the Mariners in the week or so before the All-Star break in mid-July.
Iwakuma struggled in his three starts before the injury, going 0-1 with a 6.61 ERA. But he'd been a key piece in the Mariners' pitching staff the prior three seasons, posting a 2.97 ERA -- the ninth-lowest mark in the Majors -- and a 37-19 record in 77 starts from the time he joined Seattle's rotation on July 2, 2012.
Roenis Elias has filled in well for Iwakuma with a 2-2 record and 3.07 ERA in seven starts, but now the Mariners are down a second starter with James Paxton on the 15-day disabled list with a strained finger tendon.
• Reliever Danny Farquhar, demoted to Triple-A Tacoma last week, started for the Rainiers on Tuesday night. But McClendon said that was more about giving the right-hander a chance to work on his pitches rather than attempting to stretch him out as a potential starter.
"It's to straighten him out, more than anything," McClendon said. "Working one inning in relief, it's hard to get straightened out. Hopefully he can work three or four [innings], get a feel for his pitches again and get it going."
• Tacoma shortstop Ketel Marte, the Mariners' No. 3 prospect by MLB.com, fractured his left thumb on Sunday and is expected to miss about six weeks. The 21-year-old was leading the Pacific Coast League in batting average (.343) and hits (61), and was second in stolen bases (17), in 51 games.
• While Mariners catcher Mike Zunino continues hitting below .200 in his second full season in the Majors, McClendon believes the 24-year-old is learning on the job and not getting down on himself.
"I think he's very resilient," McClendon said. "This is a tough learning curve for Zunino. I keep reminding everybody, he had 300 at-bats in the Minor Leagues. There's going to be some growing pains and it's not going to look pretty from time to time. But he's a very bright kid, a tough kid. He'll get it."
• Right-handed reliever Mayckol Guiape was optioned to Tacoma on Tuesday morning to make room for Tuesday starter Mike Montgomery on the 25-man roster. That put Seattle's bullpen back to seven relievers after Monday's temporary promotion of Guiape, who threw 2 1/3 perfect innings in his MLB debut in Monday's 7-2 loss to the Yankees.
• Left-hander Tyler Olson was activated off the 15-day DL (bruised knee) on Tuesday and optioned to Tacoma. Olson pitched out of the bullpen in 11 games for Seattle at the start of the season, but will be worked into the rotation for the Rainiers and is slated to start Friday against New Orleans at Cheney Stadium.