EUGENE, Ore. (AP) -- Jeremiah Johnson found added motivation against Division I-AA Portland State after seeing a teammate get hurt in the first quarter.

Replacing injured star Jonathan Stewart, Johnson rushed for 86 yards and three touchdowns to lift No. 25 Oregon to a 55-12 victory over Portland State on Saturday.

Stewart suffered an abdominal strain after his first carry when he was dragged well out of bounds by Portland State safety Charles Manigo.

No flags were thrown on the play, prompting a chorus of boos from the Autzen Stadium crowd.

"It fired me up because it was unnecessary," Johnson said. "That's like a brother to me. It came to be a little personal, instead of just going out there and playing a ballgame. (Stewart) just told me to go out there and go get 'em."

Johnson did, scoring on runs of 1, 6 and 1 yards in the first half to help the Ducks (6-2) overcome Stewart's injury and an early surge from the Vikings (5-4).

Stewart stayed down for about a minute as trainers attended to him. He walked off the field on his own strength and came back later in the quarter for one final carry.

He did not return after that.

Oregon coach Mike Bellotti said he was "livid" that no penalty was enforced.

"That play happened 10 yards out of bounds," he said. "I'm very upset with the officials. I'm going to turn that play in, I'll tell you that right now."

Stewart said Manigo apologized after the game.

Despite Stewart's absence, Oregon seemed to be in control midway through the first quarter after jumping out to a 14-0 lead.

Portland State's Mu'Ammar Ali kept it close, rushing for 127 yards in the first half, including a 57-yard TD run to bring the Vikings within 21-12 early in the second quarter.

Johnson quickly answered with two touchdowns in the final 4:15 of the second quarter, helping Oregon to a 35-12 halftime lead.

"It's always good to see a fellow running back do his thing out there," Stewart said. "I just told him to go out there and dominate."

Benched at halftime of Oregon's 34-23 loss at Washington State a week ago, Dennis Dixon rebounded with a solid effort against the Vikings, completing 13 of 23 passes for 150 yards and one TD. He also rushed for 42 yards on six carries.

Oregon's Jaison Williams had eight catches for 112 yards and a touchdown, his fifth 100-yard receiving game of the season.

Ali finished with 139 yards on 23 carries.

Dixon capped an eight-play, 72-yard drive with an 11-yard toss to Williams in the back of the end zone, giving the Ducks a 7-0 lead early in the first quarter. Dixon was 6-for-6 on the drive for 50 yards.

Portland State receiver Tremayne Kirkland took a quick pass from Sawyer Smith and ran in for a 19-yard score late in the first quarter, cutting the Vikings' deficit to 14-6.

Backup QB Brady Leaf's 3-yard scoring run pushed the Ducks ahead 21-6 early in the second quarter, but Ali answered with his 57-yard run. The Vikings failed on a 2-point conversion try.

The Oregon defense made it a route in the second half, limiting the Vikings to 62 yards in the final two quarters.

Portland State never advanced past its own 46 in the second half. Oregon forced four Portland State turnovers and had five quarterback sacks in the game.

"We knew we had to play a perfect game," Portland State coach Tim Walsh said. "There were too many mishaps. We just had our back up against the wall too many times."

Paul Martinez kicked two second-half field goals, A.J. Tuitele returned an interception 26 yards for a TD and Andiel Brown scored on a 7-yard run to cap the scoring for the Ducks.