Oregon quarterback Marcus Mariota, who was wearing a brace during the Ducks' 26-20 loss to Stanford on Thursday night, has a partial sprain of the MCL in his left knee, a school official acknowledged to Ivan Maisel of ESPN.com.
Mariota, a Heisman Trophy contender, said he was playing on a left knee that was a "little banged up." He finished with 20-of-34 passing for 250 yards and two fourth-quarter touchdowns as the Ducks (8-1, 5-1 Pac-12), No. 3 in the BCS standings, fell short at No. 5 Stanford (8-1, 6-1).
Television station KVAL of Eugene, Ore., initially reported before Thursday's game that Mariota was wearing the brace because of the MCL injury.
Mariota entered Thursday's game with 56 carries for 511 yards, an average of 9.1 yards per carry. But the redshirt sophomore rarely moved out of the pocket in the first half, and when he tried in the second half with Oregon down 20 points, the Stanford defense made him pay with a sack and fumble at the Cardinal 28.
Mariota had to be helped off the field with what appeared to be a left knee injury, but he returned for the Ducks' next offensive series.
Asked after the game how much pain he was in, Mariota said, "I don't know. I tend to just not focus on the pain because you've got to play with it."
Mariota ran the ball three times and was sacked three times. He finished with minus-16 yards.
Ivan Maisel of ESPN.com contributed to this report.