Coach Mike Riley made the announcement after Monday's practice, in which Mannion took all the snaps with the first unit.
"Obviously, we feel great about our quarterback situation,'' Riley said, according to the Oregonian. "It's a tough decision to make.''
Cody Vaz went 2-0 as the starter in Mannion's absence, completing 62.1 percent of his passes with three touchdowns and no interceptions.
"I obviously want to be out there, I want to be playing,'' Vaz told the Oregonian. "It's going to be tough, but I'm going to handle it the best I can and try to support Sean.''
Mannion has completed 63.3 percent of his passes with seven TDs and four interceptions. Vaz has a slightly higher passing efficiency rating.
Oregon State is 6-0 and is No. 7 in this week's BCS standings.