The Mariners scored two runs or fewer in 16 of Fister's 21 starts this season and just once exceeded five runs for the hard-luck starter. But they rewarded the rookie Beavan with seven runs in the first five innings on a night they finished with 16 hits and their third win in the past five games.
Beavan, who had previously started four times in place of the injured Erik Bedard, improved to 2-2 with a 3.24 ERA, giving up three runs on seven hits in 6 2/3 innings.
The 22-year-old Texan took a line shot off his left ankle on his fourth pitch of the game by Jemile Weeks, and the ball bounded over to third baseman Chone Figgins as the speedy leadoff hitter legged out a base hit.
But after the A's loaded the bases with no outs, Beavan wiggled out of the jam by striking out Josh Willingham and Ryan Sweeney before getting Conor Jackson to fly out to left. He limited Oakland to just one hit over the next five frames until the A's pushed across a pair of runs on three hits in the seventh.
The Mariners jumped on Trevor Cahill for five runs in the second inning, capped by a bases-loaded triple by rookie Dustin Ackley, whose line drive skipped over the glove of sliding right fielder Ryan Sweeney.
Ackley, who continued his extra-base assault with a double in the fourth before scoring on a Miguel Olivo, raised his average to .311 with a 2-for-4 night and three RBIs.
Ackley has 19 extra-base hits in his first 37 games. Ken Griffey Jr. had 18 extra-base hits in his first 50 games as a rookie. Alvin Davis holds the franchise record with 26 in his first 50 games.
Figgins started at third base and singled and scored in his first at-bat, but left the game in the third after suffering a hip flexor injury. He was listed as day to day.
Justin Smoak, originally slated for a second straight day off, replaced Figgins -- with Adam Kennedy sliding back over to third -- and wound up going 2-for-3 with a double, a walk and two runs after hitting just .141 for the month of July.
Left fielder Casper Wells, obtained from Detroit in the Fister trade, started his second game for Seattle and went 2-for-4 with a run scored as every Mariners starter except Kennedy wound up with a hit.