Parker has given up one run or fewer in his last four starts, seven of his last eight, and 10 of 14 in his career, dating back to Sept. 27, when he made his big league debut for Arizona.
Parker is one of only two pitchers since 1900 to give up one run or fewer in 10 of his first 14 career starts, joining former New York Giants pitcher Ferdie Schupp, who did it from 1913-17.
"For a guy that didn't even start with us, he's been about as consistent as you can be," A's manager Bob Melvin said. "We would've liked to have rewarded his win-loss record a little more, but yeah, based on what you saw in Spring Training and where [he and fellow rookie Tommy Milone] are right now, absolutely you've got to be happy."
The Mariners will counter with lefty Jason Vargas, who has been no stranger to the A's this season. Vargas will be making his fourth start against Oakland this year, going 1-0 with a 2.45 ERA, giving up just 10 hits in 18 1/3 innings.
Although Vargas hit a rough stretch in the middle of June with three straight losses, the southpaw has bounced back nicely in his last two outings -- one of which was against Oakland on June 26, when he struck out a career-high-tying 10 batters over 6 2/3 innings.
In his last outing, he pitched eight innings of one-run ball against Boston and will look to duplicate his last two outings when he toes the rubber on Saturday.
"I can't say enough about his effort," Mariners manager Eric Wedge said. "It's always a good thing to see a starting pitcher go deep in the ballgame, but when your club really needs it, when your bullpen really needs it, to see a starting pitcher step up like that, that says a lot to me."
Mariners: Montero remains day-to-day
Catcher Jesus Montero remains day-to-day with a mild concussion suffered on Wednesday against the Orioles. Montero did not play Friday and will be reevaluated Saturday, but Wedge said the backstop won't go on the seven-day disabled list because the All-Star break is coming up.
Mariners first-round selection Mike Zunino was named the winner of the 2012 USA Baseball Golden Spikes Award, which is given to the nation's top amateur baseball player. The Mariners selected Zunino third overall in the First-Year Player Draft last month.
Athletics: Suzuki, Carter to start Saturday
Catcher Kurt Suzuki will get the start behind the plate in Saturday's matchup while Chris Carter will get the nod at first. With the A's utilizing a platoon at first base recently, Carter -- Friday's hero with a walk-off, three-run homer in the 11th inning -- will get the nod over the left-handed hitting Brandon Moss because the Mariners are starting a southpaw on the mound. Carter is 3-for-8 with a home run and a double against left-handed pitchers this season.
Carlos Peguero started in right field for the Mariners on Friday in his first game since being called up from Triple-A. Wedge said he will try to get Peguero as much playing time as possible this weekend.