Fuentes will shift to setup duties, while Balfour will be the team's closer.
"I told Brian, that for now, until he has a few good outings in a row, that I would stick with Balfour," manager Bob Geren said before Tuesday's game in Anaheim.
The switch follows a rough stretch for Fuentes, who has been charged with the loss in each of his past four games. Owner of a 1-7 record and 5.06 ERA, the veteran southpaw is the first reliever to have seven losses before June 1 since Gene Garber also tallied that many in 1979 with Atlanta, according to the Elias Sports Bureau.
Balfour, meanwhile, had not allowed a run in 14 games -- spanning 14 1/3 innings -- until Sunday, when he offered up a game-tying home run to the Giants' Nate Schierholtz in San Francisco.
The gritty right-hander has not allowed a run in 19 of his 22 appearances this season and has surrendered just five total in 21 2/3 innings for a 2.08 ERA. He's tied for fourth among American League relievers in strikeouts (26) and has held opponents to a .192 batting average.