Iwakuma, 30, was a starter during an outstanding 10-year career in Japan, but he has pitched exclusively in the bullpen this season in his first year in the Majors.
The right-hander is 1-1 with a 4.75 ERA in 14 appearances. His ERA is 3.18 when he's pitched three-plus innings, which he's done five times.
Millwood felt some pain in his previously injured groin during his last start Wednesday against the A's and was removed after 2 2/3 innings. He did some light throwing again Friday and seems to be doing well, but the Mariners will be cautious after he injured the same area on June 8 after throwing six no-hit innings against the Dodgers.
Manager Eric Wedge said Millwood likely would be pushed back to next weekend's three-game series in Oakland.
"We didn't want to put him on the DL, because we didn't want him going that long without pitching," Wedge said. "He's been throwing pretty good and we didn't want him to lose that feel, with the All-Star break coming, too."
With Iwakuma removed from the bullpen equation for now, the Mariners recalled Steve Delabar from Triple-A Tacoma to bolster their relief crew.