At 9:45 a.m. on Monday morning, just as the team was about to go into a meeting, the news hit.
"It was a total shock," Riley said. "The doctor and the trainer came running down the hall. So I knew it wasn't good."
Mannion is expected to have surgery to repair meniscus damage in his left knee as early as Wednesday. He will be week-to-week following the procedure.
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.
A classic drop-back passer with a calm demeanor and a good grasp of the fundamentals, Mannion is the son of a high school football coach. He is one of four team captains for Oregon State this year.
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.
Vaz grew up in Lodi, Calif., and played at St. Mary's High School, where he was 24-5 in two seasons as the starter. He was the Stockton Record's player of the year his senior season.
Vaz's backup going forward will be redshirt freshman quarterback Richie Harrington, a walk-on.
Oregon State has not opened a season 5-0 since 1939.