Seattle Mariners right-hander Hisashi Iwakuma will not participate in next year's World Baseball Classic, Japan's WBC skipper, Koji Yamamoto, revealed Sunday.

Iwakuma, 31, just completed a two-year, $14 million extension to his contract on Nov. 2. The team expects him to be one of the pillars of next season's starting rotation behind ace Felix Hernandez. An informed source said the pitcher wants to give priority to his preparations for next season.

"He thought about it long and hard, but figured he has a responsibility to give priority to getting his body in shape to pitch a full season in 2013," the source said.

Last offseason, Iwakuma joined Seattle as free agent from the Rakuten Eagles. He went 9-5 with two saves and a 3.16 ERA in 30 appearances, including pitching in middle relief this past season after getting off to a slow start in spring training.

Iwakuma was instrumental in helping Japan win its second straight WBC title in 2009.

Yamamoto confirmed he had received word from Iwakuma that the right-hander had joined Texas Rangers pitcher Yu Darvish in opting out of the WBC. The skipper also said "chances are high" that free agent right-hander Hiroki Kuroda would follow suit.

"It's a bitter decision for him (Iwakuma)," Yamamoto said. "It's the same situation with Darvish. We just have to deal with it."

"Our pitching staff is going to be exclusively 'Made in Japan.' So that's the way we have to go."