Oregon State quarterback Sean Mannion will miss the No. 10 Beavers' game at BYU on Saturday, and is out indefinitely with a knee injury. "It's very, very disappointing for the young man. He's invested a tone and he's going to miss some time," coach Mike Riley said Monday. "He'll be back and he'll be fine." Mannion is expected to need surgery but the nature of the injury wasn't discussed by Riley other than to say he'll let Mannion and team doctors talk about it in due time. Mannion has completed 63 percent of his passes for 1,358 yards this season. He threw four interceptions last week but the Beavers beat Washington State to improve to 4-0 overall and 3-0 in the Pac-12. He was injured on a routine handoff. Oregon State will start junior Cody Vaz against BYU. "Cody is a good quarterback. He's been preparing for this for a long time. Cody's had a ton of reps," Riley said. Reps count for something, but what he lacks is game experience. Vaz hasn't played since the 2010 season, when he made appearances in five games and completed 6-of-17 passes for 48 yards. "Being the backup, you have to prepare yourself," Vaz said. "You have to have a good mental approach each week, and that's something I've been doing these first four games, is preparing myself like I'm the starter. Now I am, and I'm going to make the most of this opportunity. I'm pretty excited ... it has been a while since I've been a starting quarterback."