I'm sure you've all heard this by now, but this comes via ESPN.

Oregon quarterback Dennis Dixon, a Heisman Trophy front-runner who had guided the Ducks to an 8-1 record and a No. 2 BCS ranking, twisted his left knee Thursday and left in the first quarter of a 34-24 loss to host Arizona.

Dixon, a big threat on the ground who also rates fourth in the nation in passing, freely roamed the sideline but did not return, a significant blow for a team averaging almost 43 points and 511 yards a game.

Dixon's left knee appeared to buckle as he tried to plant on an option carry with about five minutes to go in the quarter.

"My foot got planted into the ground and my foot went the other way," Dixon told ESPN's Erin Andrews late in the fourth quarter.

Dixon sprained the same knee against Arizona State on Nov. 3. He wore a brace for Thursday night's game.

Dixon had a 39-yard touchdown run on the Ducks' first possession. He was 5-of-8 for 62 yards and threw an interception in the end zone.