The 34-year-old -- recovering from a strained lat muscle behind his right shoulder -- allowed just one run, one walk and scattered five hits while striking out six hitters in a 5 2/3-inning, 82-pitch (57 strikes) outing in an 8-4 win for Triple-A Tacoma over the Las Vegas 51s.
He got ahead in counts, reached three-ball counts against just three of the 24 hitters he faced and flashed far more efficiency than he did in a four-inning, 68-pitch start for the Rainiers on Thursday night. But his night soured when he was forced to leave in the sixth with what he said was a right middle finger blister.
He struck out the first two hitters he faced in the sixth inning before allowing a walk to Las Vegas left fielder Brandon Allen. He then threw one pitch -- a strike -- to right fielder Travis Taijeron before he was lifted for reliever Danny Farquhar.
The right-hander was not wearing any bandages on his right hand after his start and said he was fine.
"It happened in the sixth inning. I threw a split and felt the blister pop, and then on the curveball where I struck [center fielder Alex Castellanos] out, I felt it kind of rip open," Iwakuma said through interpreter Antony Suzuki. "It's common for me."
Thoughout his first few innings, it appeared Iwakuma would easily reach his target of six innings before reaching his 85-to-90-pitch limit. He allowed at least one hit in four of his first five innings but was able to keep Las Vegas off the scoreboard aside from a single run on back-to-back hits to first baseman Brooks Conrad and Castellanos in the fourth. Through four innings, he had thrown just 48 pitches, compared with 68 in his previous rehab start.
"He was phenomenal," Rainiers manager Pat Listach said. "It's up to the big league club what they're going to do with him next. Whether his next start will be here or in Safeco, we don't know, but he was really good."
Iwakuma's fastball reached 90 mph with good movement. He was also able to command his breaking pitches more effectively than he did on Thursday.
"He was sharper," Listach said. "He changed speeds on his fastball. That's promising and that's what a big league pitcher does, he gets you looking for one pitch and he changes speed on it."
Prior to the Mariners' 5-0 win in San Diego on Tuesday, manager Lloyd McClendon said the team would make its decision on what to do with its rotation sometime in the next couple days based on Iwakuma's performance.
On Tuesday, Iwakuma exited his start confident his blister would not lead to a setback and confident he could handle an increased pitch count.
"I'm able to make my adjustment with the two trainers they have up in Seattle. They know the situation as well, so I should be fine," Iwakuma said. "I feel comfortable, mentally and physically."