The two will play with mostly pride on the line - and perhaps redemption - when the No. 12 Ducks host the Beavers in the annual Civil War rivalry game on Friday afternoon.
Mariota, who took a hard hit to the head in last weekend's 42-16 loss to Arizona, says he has passed all of the concussion tests and will play. But there are still lingering questions about the health of his left knee.
Mannion's issue seems to be confidence after throwing 10 interceptions in Oregon State's last three games - all losses. He threw three in the Beavers' 69-27 loss to Washington last weekend.
''He's actually good. I think he's pretty much a realist. He wants to do well and he works real hard at it,'' coach Mike Riley said Tuesday. ''He's kind of always the same. He comes back to work, he starts learning. He spends extra time up here when he has it in the afternoons just getting ready to go. I think he's great that way.''
While Mannion's had a rough recent run, overall he's on track for a record-breaking season for the Beavers (6-5, 4-4). He's ranked second nationally with an average of 371.7 yards passing, and fourth with an average of 31.5 competitions per game.
He's got Oregon State's single-season record of 34 touchdown passes already this season, which also ranks him second among FBS quarterbacks behind Fresno State's Derek Carr.
The Beavers dropped their opener to Eastern Washington before reeling off six straight wins to become bowl eligible. But with four straight losses, the postseason is now in jeopardy: Nine Pac-12 teams are eligible with just seven bowl agreements. Those teams on the outside will have to see if they're extended invitations.
A drop-back pocket passer, Mannion was getting some Heisman buzz - along with Mariota - earlier this season before the losses piled up. His favorite target, Brandin Cooks, leads the nation with 141.8 yards receiving.
Mariota, meanwhile, is known for his scrambling ability as much as his arm - but he's been hampered in recent weeks by what appears to be a knee injury he sustained in the first half of Oregon's victory over UCLA on Oct. 28. He had negative yardage on the ground, a result of six sacks, against Stanford and Utah.
He has said that he's not taking off as often because of what defenses have been throwing at him, insisting that the knee isn't an issue. There's no way to tell how serious it is, because Oregon doesn't discuss injuries as a policy.
But the attention shifted from the knee to his head this week. Mariota said he got his ''bell rung'' while trying to tackle Arizona's Shaquille Richardson on an interception late in the loss to the Wildcats.
He appeared dazed after the hit, and was surrounded by trainers on the sidelines after he was helped off the field. It was the second of two interceptions he threw in the game, his first two picks of the season. Later, he said he'd never suffered a worse loss.
Overall, Mariota is averaging 284.3 yards passing per game with 27 total touchdowns. He's also run for 529 yards and nine scores.
The defeated Ducks (9-2, 6-2) dropped from No. 5 to No. 12 in the AP Top 25 poll. The chance they had at a national championship bid was already skewered by a 26-20 loss to the Cardinal, but the loss to the Wildcats took them out of a shot at the Pac-12 championship and the Rose Bowl.
Now the Ducks are most likely looking at the Holiday or the Alamo bowls, ending a streak of four straight BCS bowl appearances.
Mariota was already facing questions this week about whether the Civil War will be his last game at Autzen Stadium. There has been speculation that the sophomore might declare for the NFL draft.
''I'm being truthfully honest, I'm not sure,'' he told reporters after practice Monday. ''After the bowl season I'll go home and just kind of talk through this with my family and see where is the best fit for my family.''
For now, he's just looking at the Civil War.
''This game does mean a lot. It means a lot to the state,'' Mariota said. ''It's an honor to play in it.''
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