CORVALLIS, Ore. (AP) - 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 and watched from the sidelines at practice on Monday. Coach Mike Riley said an MRI has indicated the 6-foot-5, 212-pound sophomore quarterback will require surgery.
``It's very, very disappointing for the young man. He's invested a ton and he's going to miss some time,'' Riley said. ``Like I told him, `You're going to be fine. I read about a guy who hurt his knee in the first game of a the season a few years ago and he's still pretty good - Tom Brady. The football stuff happens, you've just got to live with it and get better from it.'
``He'll be back and he'll be fine.''
The specific nature of Mannion's injury was not released. Mannion lest practice without speaking to reporters and Riley said he would undergo additional examination Monday afternoon.
There was no timetable for the surgery or his return.
``I'm going to let the doctors talk about that, or Sean, when he's ready for that,'' Riley said.
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.
He has thrown for 1,358 yards with seven touchdowns and four interceptions this season. He is sixth on Oregon State's career list with 4,686 yards.
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, adding that he and Mannion and nearly equal reps in fall camp.
``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.
``You have to prepare yourself. You have to have a good mental approach,'' Vaz said. ``That's what I've been doing for the past five weeks like I'm the starter. And now that I am I've got to make the most of this opportunity.''
Vaz's backup will be redshirt freshman quarterback Richie Harrington.
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. Oregon State has not opened a season 5-0 since 1939.