"Might have been one of his best outings of the year," manager Scott Servais said.
The key was Iwakuma's fastball.
"I was able to command my fastball, and my fastball had more life than usual," Iwakuma said through interpreter Antony Suzuki. "So I was able to get strike one."
It was a stark contrast from his last outing in Chicago. Nothing was going right for him against the Cubs. Iwakuma said his arm and his balance felt off. He allowed five runs in just three innings in his worst outing of the season.
"We got him out early, hoping he could bounce back," Servais said. "And he certainly did. It's great to see."
Iwakuma worked his mechanics and hit the weight room when he got back. On Tuesday, Iwakuma threw 73 strikes, more than he has in any other start this season, and only 24 balls.
"There were a lot of lefties in there," catcher Mike Zunino said. "He wanted to use the inside of the plate, and he was able to use that slider down and in, and a little shorter slider up in their hands. He was able to keep them at bay with that and then use that splitter when he needed to."
Iwakuma faced the Red Sox once before this season, in Boston on June 17. He got the win then too, but gave up four runs, including a two-run homer to David Ortiz.
"They've hit me pretty well in the past," Iwakuma said. "But the last time I faced them in Boston, I had a good idea of what I wanted to do today."
His game plan worked.
Maddie Lee is a reporter for MLB.com based in Seattle.