Oregon State's enthusiasm over its 4-0 start has been tempered by the news that starting quarterback Sean Mannion will undergo knee surgery.

Beavers head coach Mike Riley told a group of reporters on Monday afternoon that the sophomore will be out an undisclosed amount of time. According to the Statesman Journal, the injury occured when Mannion was making a handoff during the Beavers' 19-6 win over Washington State on Saturday. He played the entire game against the Cougars but threw three interceptions in the contest.

Riley has the Beavers off to a 4-0 start, and the aerial attack led by Mannion has been a major reason for the team's success. He ranks sixth in the nation at 339.5 passing yards per game and was particularly strong in the team's wins over Wisconsin, UCLA and Arizona.

In four games this fall Mannion has thrown for 1,358 yards and seven touchdowns.

Junior Cody Vaz will likely take over the starting job in Corvallis. He hasn't played since his freshman year of 2010, when he completed just six of his 17 attempts for 48 yards.

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