CORVALLIS, Ore. -- Oregon State quarterback Sean Mannion is out indefinitely after injuring his left knee during the No. 10 Beavers' victory over Washington State on Saturday.
Backup Cody Vaz will start for the undefeated Beavers when they play a nonconference game against BYU this weekend.
Mannion had his knee in a brace at practice on Monday. Coach Mike Riley said an MRI has indicated the 6-foot-5, 212-pound sophomore quarterback will require surgery.
The specific nature of Mannion's injury was not released.
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"We just found out this morning," Riley said. "They did an MRI this morning."
Mannion threw for 270 yards and a touchdown in Oregon State's 19-6 victory over the Cougars. He apparently injured the knee on a handoff but did not leave the game.
Following the victory, Oregon State jumped into the Top 10 of the AP rankings for the first time since 2001, when the Beavers went to the Fiesta Bowl.
Mannion was averaging 339.5 yards passing a game, second in the Pac-12 and sixth in the nation.
Mannion has thrown for 1,358 yards with seven touchdowns and four interceptions this season.
Vaz, a 6-foot-1, 198-pound junior, played in five games in the 2010 season, completing six of 17 passes for 48 yards. He did not appear in any games last season for the Beavers.
Riley said he has confidence in Vaz.
"Cody is a good quarterback. He has been preparing for this for a long time," Riley said. "He'll be good and we'll be ready to go."
Vaz said he is excited to make his first start.
"I'm just going to prepare myself to keep the streak going," he said.
Mannion was winning accolades for his play this season. He is a classic drop-back passer with a calm demeanor and a good grasp of the fundamentals, the product of growing up the son of a high school coach.
Last season as a redshirt freshman, Mannion unseated junior starter Ryan Katz, who had started the 2010 season for the Beavers. Katz has since transferred to San Diego State.
Mannion is one of four team captains for Oregon State this year.
The Beavers have rebounded from going 3-9 last season to opening 4-0 this season.