But Montgomery, called up a month ago to fill in for the also-injured James Paxton, has been magnificent in six starts with a 1.62 ERA and has thrown back-to-back shutouts. The Mariners could hold off on the decision by having Iwakuma make another start with Triple-A Tacoma, particularly since he developed a blister on his right middle finger late in Tuesday's 5 2/3-inning start.
The other scenarios would be to send Roenis Elias back to Tacoma, move J.A. Happ into the bullpen or go at least briefly with a six-man rotation heading toward the All-Star break in two weeks.
Manager Lloyd McClendon declined to speculate on any of those possibilities prior to Wednesday's series finale against the Padres.
"It's nice to have tough decisions to make," he said. "That means you're pretty good. From that standpoint, we have depth. We'll sit down and hash it out and figure it all out."
McClendon expected to discuss the situation with general manager Jack Zduriencik and pitching coach Rick Waits during the team's flight to Oakland on Wednesday evening.
Iwakuma has gone 1-0 with an 0.93 ERA in two Tacoma starts and also gave up just one run in 3 2/3 innings in an initial start with Class A Everett.
"He did OK. It was better than the last outing," McClendon said of Iwakuma's start on Tuesday, when he allowed five hits and one run with six strikeouts in an 8-4 win over Las Vegas. "The velocity got better each outing. I know he touched 91 last night, and that's a good sign. He's moving in the right direction."
One issue could be the blister, but Iwakuma deals with that problem nearly every time he builds his pitch count up early in seasons due to the pressure the split-finger fastball puts on his middle finger.
"That certainly will be part of the evaluation, and then we'll go from there," McClendon said. "I don't want anybody pitching around anything. We need him 100 percent, and hopefully he will be 100 percent."
The Mariners may want to see how Elias and Happ pitch in the first two games of the Oakland series before making any decisions. Elias is 4-5 with a 4.25 ERA in 12 starts since stepping in for Iwakuma on April 26. He pitched extremely well early, but is 1-2 with a 10.29 ERA In his last three outings. The 26-year-old has Minor League options and could be sent down to Tacoma without being exposed to waivers.
Happ doesn't have any Minor League options, but does have some experience pitching out of the bullpen in his nine-year Major League career. Happ (3-5, 3.89 ERA in 15 starts) also pitched better early this season and has now lost four straight decisions and gone nine starts since his last win.
But Happ has thrown better than his results would indicate, as the Mariners have totaled just 22 runs in those nine games -- and just six in his last five starts -- while he's posted a 4.38 ERA while going 0-4 with five no-decisions.
• Three Tacoma players -- shortstop Ketel Marte, first baseman Jesus Montero and right-hander Forrest Snow -- were selected Wednesday for the Triple-A All-Star Game on July 15 in Omaha, Neb. The 5 p.m. PT game will be televised on the MLB Network.