Iwakuma, who missed the entire spring with a sprained tendon in his right middle finger, gave up six hits and three runs (two earned) in his four frames for Tacoma.
"I'm prepared to go Friday," Iwakuma said through translator Antony Suzuki. "They haven't told me anything yet, so we'll have to wait and see. But physically, a four-day rest would be good, as always."
Said manager Lloyd McClendon: "He'll pitch somewhere Friday. I'm not sure where yet."
McClendon said Sunday's rehab outing went about as expected for Iwakuma's first official game action after he threw several simulated outings against teammates.
"He threw OK; he got his pitch count up to 86, which is the most important thing," McClendon said. "His command was OK -- not great, but I didn't expect it to be. What I wanted was just him coming out feeling good and healthy from the outing, and we accomplished that."
Iwakuma said his command wasn't quite where it's needed yet, not surprising given the lengthy layoff.
"It was OK," Iwakuma said. "So-so; not 100 percent yet. It was my first game in a long time, and I was trying to get my feel back for the game. I need some more time, but it's going to be there."
The Mariners are tentatively listing Brandon Maurer for Saturday's game in Houston and Chris Young for Sunday, with a TBA for Friday's series opener.
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.