Oregon State's rise to No. 10 in the rankings has been one of the surprise stories of the college football season, but the loss of sophomore quarterback Sean Mannion will jeopardize the Beavers' standing.

Coach Mike Riley announced on Monday that Mannion will have knee surgery and will be out indefinitely. Mannion sustained the injury during Saturday's win against Washington State. The extent of the injury and how long he will be sidelined is not known.

"We just found that out this morning," Riley said Monday, according to the Corvallis Gazette-Times. "Sean is not going to play. He's going to have a surgery and will be out for an undetermined amount of time."

This is a tremendous blow to the Beavers. Mannion had been outstanding in leading the Beavers to their 4-0 start, and he ranks sixth in the nation in passing offense.

Mannion is the only Oregon State quarterback to have thrown a pass this season. Junior Cody Vaz and freshman Brent VanderVeen are the other scholarship quarterbacks on the roster. Vaz is likely to start for the Beavers in Saturday's game against BYU.

Ryan Katz, who was Oregon State's starting quarterback at the beginning of last season, transferred to San Diego State and is the Aztecs' starting quarterback this season.