CORVALLIS — With so many scenarios for how the season may — or may not — play out, No. 5 Oregon is focused only on Oregon State and the 116th Civil War.

The Ducks’ march toward the national championship game detoured last Saturday with a 17-14 overtime loss to Stanford. Now, even the team’s shot at the Pac-12 championship is in jeopardy.

“You have to be mentally tough," Oregon running back Kenjon Barner said. “It’s football. You’re going to lose some games. It happens."

Today’s Civil War at Oregon State’s Reser Stadium is a must-win game for Oregon (10-1, 7-1) if they are to keep alive their hopes for a fourth straight Pac-12 title. But the Ducks will need UCLA to help with a victory over Stanford in Los Angeles. To be back in the mix for a national championship berth, they will likely also need USC to topple undefeated and top-ranked Notre Dame.

Oregon State coach Mike Riley laughed when asked if he is worried the Ducks will be doubly motivated against the 16th-ranked Beavers (8-2, 6-2) after last week’s loss.

“I don’t know what their mindset is going to be," Riley said. “But they’re a really good, resilient football team. So that’s what we expect."

No matter what happens today, Oregon State can count this season as a success. The Beavers have staged a stunning turnaround with essentially the same team that went just 3-9 last season.

Oregon State’s best weapon against the speedy Ducks may be its defense — much like Stanford’s was in the Cardinal’s victory over Oregon last weekend.

The Ducks have the fourth-ranked offense in the country, averaging 548.3 yards per game, and the fifth-best rushing offense with 313.5 yards a game. Stanford’s stout defense was able to hold the Ducks to 198 yards on the ground and 405 yards in total offense.

The Cardinal held Barner, averaging 136 yards rushing going in, to just 66 yards.

Oregon State has the nation’s No. 14 rushing defense, allowing opponents only 108.7 yards on the ground per game. The Beavers’ overall defense is ranked third in the Pac-12, allowing an average of 345 yards.

On offense, Oregon State will start Sean Mannion at quarterback because quarterback Cody Vaz returned to practice only Wednesday after a left ankle sprain.

Mannion started the first four games of the season, passing for seven touchdowns and averaging 339 yards, but he injured his left knee and required surgery. Vaz, who had not started a game since high school, took over and helped the Beavers to win in the next two games, and later became the team’s starter.

But Vaz sprained his left ankle in the final moments of a loss to Stanford two weeks ago, and he sat out last Saturday during Oregon State’s 62-14 victory at home over California.

Oregon defensive coordinator Nick Aliotti said this week that Oregon State’s standout receivers, senior Markus Wheaton and sophomore Brandin Cooks, have the Ducks’ attention.

Cook has 1,039 receiving yards with five touchdowns this season, and Wheaton has 986 yards with 10 touchdown catches. Both are among the top 15 in the nation for average receiving yards.

“I wish I could tell you ‘Not many worries, not many concerns,’ but we got our work cut out for us because they remind me of an ’SC down-the-field throwing team, and last time I checked we didn’t do really well in that one," Aliotti said. “We’re going to have to cover those receivers."

For the record, Oregon beat USC 62-51, but Trojans quarterback Matt Barkley passed for five touchdowns, including two to Marqise Lee.

Oregon quarterback Marcus Mariota spent last Sunday mulling over the loss to Stanford then got back to work on Monday. The redshirt freshman passed for 207 yards and a touchdown against the Cardinal.

“There were a lot of times I was trying to force it too much," Mariota reflected this week.

Mariota needs a total of three touchdowns — via pass or run — to reach Oregon’s single-season record of 36 set by Akili Smith in 1998 and matched by Darron Thomas in 2011. He already holds the Pac-12 freshman record with 29 touchdown passes this season.

In what could be his final game at Autzen Stadium, Barner needs two rushing touchdowns to match pal LaMichael James’ single-season record of 21 at Oregon in 2010. He needs just 15 more rushing yards to move past Derek Loville (1986-89) into second on the Oregon career list.

The Ducks hold a 59-46-10 advantage in the Civil War, and they have won the past four games in the series, which dates back to 1894.

Today’s game will mark the fourth time that both teams are ranked for the Civil War. The last time was in 2009, when Oregon was No. 7 and Oregon State was No. 13. That game was dubbed the “War of the Roses" because the winner was guaranteed a Rose Bowl berth. Oregon won, 37-33.

Riley has seen his share of Civil Wars, both as coach of the Beavers and as the son of former Oregon State assistant coach Bud Riley, who passed away earlier this year. Growing up in Corvallis, Riley has watched the game both from the stands and from the sidelines.

“I feel like this is to be embraced. It’s fun to be involved in it, that’s one of the things about tradition and history," Riley said. “This game has been played a long time. In the record books when you total them all up it means the same thing, a win or a loss. But for the moment when you’re in it, getting to play in a fun game like this, it means a lot to everybody."