Iwakuma, 34, hasn't pitched since April 23 after feeling soreness behind his throwing shoulder in his third start of the season. The 2013 American League All-Star was expected to be a big part of Seattle's rotation again this season after going 37-19 with a 2.97 ERA in 77 starts from 2012-14, but instead he went 0-1 with a 6.61 ERA before going on the 15-day disabled list.
Manager Lloyd McClendon said Monday's simulated outing went well.
"He threw three innings and each inning went better," McClendon said. "He'll go to Everett to begin his rehab. Hopefully we'll get 40-50 pitches out of him and then re-evaluate after that."
Everett opens its season Thursday at home against Eugene and the AquaSox roster will include outfielder Alex Jackson, the club's No. 1 ranked prospect by MLB.com, as well as Oregon State right-hander Andrew Moore, the team's second player selected in last week's Draft.
Iwakuma is slated to pitch against Eugene in Saturday's 6:05 p.m. PT game. He'll likely make another rehab start or two with Triple-A Tacoma before being ready to rejoin the Mariners in early July if all goes well.
• Left-hander James Paxton, sidelined since May 28 with a strained tendon in his middle finger, has been cleared to begin playing catch late next week, McClendon said. Paxton will need to progress through the normal throwing program to build his arm back up and doesn't figure to be back until late July at the earliest.
• Mark Trumbo was out of the lineup for a second straight day Monday due to back spasms, though the Mariners new designated hitter/outfielder said he felt better and was hopeful of being available to pinch-hit if needed.
• Veteran reliever Kevin Gregg opted out of his Minor League contract with the Mariners on Monday and is now a free agent. The 36-year-old signed with Seattle on May 20 after being released by the Reds and posted a 2.89 ERA in 9 1/3 innings over seven appearances.
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.