The 34-year-old right-hander lasted five-plus innings and struck out three without allowing a walk in his return from 11 weeks on the disabled list with a strained right lat muscle, but allowed four solo home runs to an offense that had 19 hits.
"Obviously giving up four home runs is not good, but bottom line, I'm healthy so that's a good sign and I was able to make good pitches in certain situations," said Iwakuma through interpreter Antony Suzuki.
Iwakuma cruised through the Tigers' lineup early, throwing just 13 pitches in the first inning and retiring the first five hitters he faced.