Kragthorpe: How will loss of QB affect No. 10 Oregon State?

In the case of Oregon State quarterback Sean Mannion's knee injury, there's no culprit, no second-guessing and no explanation.

Nothing like what happened to BYU's Taysom Hill, in other words.

Mannion's injury could become one of the Pac-12's biggest stories this season — or not, if backup Cody Vaz performs well and the surprising, No. 10 Beavers survive upcoming games against BYU and Utah.

Mannion is expected to return at some point. For now, the news certainly is deflating in Corvallis, especially considering the mysterious way Mannion was hurt.

Hill was running on a play when he could have knelt to run out the clock, and his knee was directly hit by a Utah State player. In contrast, Mannion merely handed off on a draw play in the third quarter last Saturday and was not touched by a Washington State defender.

OSU coach Mike Riley labeled it "a nondescript play … a nothin' play," but Mannion's knee somehow was twisted. He played the rest of the game, so the diagnosis Monday came as "a total shock," Riley said.

Mannion's absence has to hurt the Beavers (4-0), who just moved into the AP Top 10. OSU is eighth nationally in passing at 339.5 yards per game, and Mannion has taken every snap because the team's biggest margin of victory is 13 points. He passed for 303 yards in a 38-28 loss to BYU last October.

OSU would have a highly experienced backup, except that deposed starter Ryan Katz transferred to San Diego State for his senior season. Yet even with Vaz not having played since 2010, when he appeared in five games and attempted 17 passes, he's apparently far more capable than his record suggests.

"We don't have to do anything differently with Cody," said Riley, citing his quick release, ability to see the field and understanding of the offense.

The 6-foot-1 Vaz is much shorter than the 6-5 Mannion, but he has played well in practice and his teammates believe in him. "They all know Cody's good," Riley said.

The other factor in the Beavers' favor is in spite of Mannion's big numbers, they're not totally reliant on him this year. OSU receivers Markus Wheaton and Brandin Cooks should help Vaz look good, while Storm Woods has given the offense a running dimension it lacked in 2011. And a defense that would have allowed exactly 500 yards to BYU last season, except that quarterback Riley Nelson lost a yard by taking a knee on the final play, is much improved.

Even having faced the potent offenses of Arizona and UCLA among their four opponents, the Beavers are fifth in the Pac-12 and 39th nationally in total defense, allowing 355.8 yards. Nelson never has beaten a top 40 defense in his college career, much less a Top 10 team.

In his return to the lineup Saturday in Provo after recovering from a back injury, Nelson will have BYU's No. 5-ranked defense on his side, challenging Vaz in his first career start.

"It's a heck of a way to break in," Riley said.

kkragthorpe@sltrib.com

Twitter: @tribkurt