OAKLAND, Calif. -- If it weren't for right-handers Hisashi Iwakuma and Kendall Graveman, change would have been the only constant for the Seattle Mariners and Oakland A's this season in terms of starting pitching.

Seattle's Iwakuma and Oakland's Graveman will face each other Saturday night at Oakland Coliseum in what shapes up as a pitching duel between rotation anchors in turbulent seasons.

Iwakuma (13-7) leads Seattle in wins and will make his team-high 24th start Saturday -- at least nine more starts than any other Mariner, including Felix Hernandez. The Mariners have used 12 starting pitchers this season.

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"He's been throwing great," Mariners manager Scott Servais said of Iwakuma on Friday before a 6-3 loss to Oakland. "He's been our most stable guy all year. We would be nowhere close to being in this position if it weren't for him. Felix (Hernandez) going down and some other guys stubbing their toe in the rotation, he has been the constant. He's been great."

Graveman (8-7) leads the A's in wins and will make his team-high 23rd start. That's two more than Sonny Gray, who's on the disabled list, and at least six more than every other A's starter. Graveman is the only member of the A's original rotation who is on the active roster, and the only one who hasn't spent time on the disabled list.

"I think Kendall's certainly taken it to a new level, and we need him to, based on the guys that we have (out)," A's manager Bob Melvin said. "It's important that he pitches like he has for us. He's basically anchoring the rotation right now, and the only guy that's still there from the beginning of the season."

Iwakuma was 0-3 in April but has gone 13-4 since then. He's 2-0 with a 2.57 ERA against the A's this season. In his past two starts, Iwakuma beat Boston and Detroit, pitching a combined 14 1/3 shutout innings with 15 strikeouts and one walk.

"It's a little different every time out there," Servais said of Iwakuma's outings. "You don't know which pitch might be working best for him or what he's going to go to. But he's been really, really a pro for us."

Graveman is 7-1 with a 3.67 ERA in his past 13 starts. He's 4-0 with a 2.25 ERA in his past five home starts, including a 3-2 victory Monday against Baltimore. Graveman allowed one run and six hits in seven innings with four strikeouts and no walks.

Graveman relies primarily on his hard sinker, but against the powerful Orioles, he mixed in more cutters, curves and changeups to keep them off balance.

Graveman said he was able to adjust on the run.

"I think it helps," Graveman said. "Going into a game there might be one game plan, but to be able to change throughout a game is another thing, and to be able to do it effectively and on the move is something that you really strive for, you really look towards. Because the game plan sounds great and dandy before the game then you get out there and have to change something up to have success.

"So yeah, I think it will help. I think it definitely brings the percentage of the fastball down a little bit from the last month, month and a half. But the game plan's still going to be the same. Go out there and compete with the fastball and locate and then make adjustments when necessary."

Graveman is 1-1 with a 3.05 ERA in five career appearances against Seattle, including three starts. This year, he's 0-1 with a 5.06 in two starts against the Mariners.