Wedge said his five-man rotation will be Felix Hernandez, Jason Vargas, Hector Noesi, Blake Beavan and Kevin Millwood.
Millwood will not be making the trip to Japan this week, however. He is staying behind because the team can only take 30 players and doesn't need all of its starting pitchers.
Hernandez and Vargas were already slated to pitch the two regular-season games against the A's on March 28-29. Wedge said Noesi will start the first exhibition game in Japan, with Iwakuma starting the second game.
Beavan and Vargas will throw a simulated game during the first workout session in Tokyo in order to keep sharp, with Beavan then being held in an emergency role in case something should happen to Vargas or Hernandez.
"Quite frankly, I think that's the best way to transition him this year, with the history he had last year with the health and what not, introducing him to this game and helping him get off to a good start," Wedge said.Iwakuma said he'd never pitched in relief except during the World Baseball Classic and some Spring Training games, but he was ready for the challenge.
"I'm really surprised about [being a] reliever, but my role is [being] one of the team, so I'll do what I have to do for the team. That's the way I think right now."
The biggest challenge could be Iwakuma's required warm-up time as someone who has typically been a starter. He was up in the bullpen throwing in the fourth inning Tuesday, then sat down and got back up again before entering in the sixth, which isn't normal for a reliever.
"That's another thing that went into our decision -- the guy loves to throw," Wedge said. "It's not a problem for him to get up and throw a little bit. And then if we call on him, he's ready. And if we don't, he's thrown a little bit anyway, which is what he would normally do.
"This is a young man who is getting stronger and working hard and as we break camp, I feel that's the best role for him. We're trying to put all these guys in the best position to succeed, not just 'Kuma."
Putting Iwakuma in the bullpen allowed the Mariners to keep Millwood after a strong spring by the 37-year-old right-hander.
"He made this ballclub," said Wedge. "He made the decision for us just by the way he pitched and competed."Even after 14 years in the Majors, Millwood was thrilled to hear the word from Wedge after throwing five innings of one-run ball against the Reds on Tuesday
"I was excited," he said. "I felt like yesterday was a big game for me. Just to be able to come in and know I had to perform to win a job and being able to do it was pretty satisfying."
Wedge said he wouldn't discuss bullpen or position-player decisions until Thursday, when the club will announce its final roster decisions before boarding the plane for Tokyo with 30 players. Of those, 25 will be eligible to play in the regular-season games.
The one move the Mariners did announce was the release of veteran reliever Shawn Camp, which Wedge said was the result of young pitchers stepping to the fore this spring.
Rookie Erasmo Ramirez and Rule 5 Draft pick Lucas Luetge are among those still in contention for the final two bullpen spots, as are Charlie Furbush and Chance Ruffin. Not everything will be clear in that regard when the team heads to Tokyo, however, since the club can bring 30 players.
The bullpen locks would seem to be Brandon League, Tom Wilhelmsen, Shawn Kelley, George Sherrill and Iwakuma, with Steve Delabar also in the mix as the team selects its seven-man relief crew.
After the two exhibition games against Japanese teams, the Mariners must then set an active roster of 28 players -- which will include Millwood, even though he's not making the trip. Then 25 of those will be chosen as the eligible players for the two regular-season games.