The nation's leading rusher went down early in the fourth quarter and trainers stabilized his arm in a sleeve as fans chanted his name. The Ducks (4-1, 2-0 Pac-12) were driving for their fourth straight touchdown in the second half against the Golden Bears (3-2, 0-2) when he was injured.
Afterward, he said that X-rays showed there was no break. He had his arm in a sling.
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True freshman De'Anthony Thomas caught a pair of touchdown passes and ran for another score for the Ducks, who have won four straight since a season-opening loss to LSU.
Giorgio Tavecchio made three field goals -- including a career-best 54-yarder -- for Cal, which led 15-14 at the half but couldn't sustain its momentum against the country's top scoring offense.
James, a Heisman Trophy finalist last season, has run for at least 200 yards in his last three games, something no Ducks player had done before.
He went into the game with an average of 153 yards rushing per game, best in the nation. He had 166 by halftime against Cal.
The crowd at Autzen Stadium fell silent when James did not get up on the fourth-quarter scoring drive, but he waved to the crowd as he was driven off the field.
He did not know his status for Oregon's game next week against No. 22 Arizona State.
"Even if I don't play we have tremendous backs," he said. "I don't think we'll miss a beat."
Cal took a 3-0 lead on Tavecchio's 27-yard field goal on the first series of the game, but James answered with a 53-yard dash for a score and the Ducks looked to be taking off. It was James' third touchdown run of 50 or more yards this season.
Tavechhio added another field goal, this time a 38-yarder, to make it 7-6. A rain storm moved in over Autzen and slowed play a bit, but Oregon extended its lead on Thomas' 17-yard touchdown run late in the first quarter.
Tavecchio's career-long 54-yard field goal cut it to 14-9.
Although it looked as though James might have been injured early in the second quarter when he was slow to get up from a hit and went to the sideline holding the back of his right thigh, he returned and had a 39-yard run, which put him over 4,000 for his career. He's the sixth player in conference history to reach that milestone.
But on the same drive, the Bears stopped Oregon on a fourth down on their own 41. Maynard led an efficient march that was capped by his 12-yard touchdown pass to Kennan Allen that gave the Bears a 15-14 lead.
After the touchdown, fans at Autzen Stadium chanted "We want Cliff!" for All-America cornerback Cliff Harris, who had only played on returns. Harris has been used only selectively by coach Chip Kelly since his return from a suspension for a speeding violation on a suspended license this summer.
Tavecchio attempted a 40-yard field goal before the end of the first half, but it was blocked.
After the half, Oregon quickly regained the lead with Darron Thomas' 23-yard pass to De'Anthony Thomas. The Ducks extended it on Kenjon Barner's 68-yard touchdown run and Darron Thomas completed pass to De-Anthony Thomas for the 2-point conversion.
Darron Thomas hit De'Anthony Thomas again with a 21-yard touchdown before James was injured on the fourth-quarter drive that was capped by Darron Thomas' 3-yard scoring pass to Lavasier Tuinei.
Darron Thomas completed 13 of 25 passes for 198 yards and three touchdowns.
Maynard finished 20 of 41 for 218 yards and a score.
Cal cornerback Marc Anthony was in street clothes after he appeared to injure his arm in the first half. The Bears would not confirm the injury.
Last season Oregon struggled against Cal last year in Berkeley, emerging with a 15-13 victory. The win featured the fewest points and yards (317) for the speedy Ducks last season. James was held to 91 yards rushing, which at that time was a season low, and Oregon had to look to its defense to keep the Bears at bay.
That game was also notable because a Cal defensive player was caught looking to the sideline after a play before abruptly collapsing to the ground with an "injury." Video of the flop went viral on the Internet.
There was widespread speculation that Cal faked the injury to slow down Oregon's high-speed spread offense. The suspicions were confirmed later in the season when defensive line coach Tosh Lupoi was suspended for a game after admitting he instructed the player to go down.
Both Cal and Oregon were coming off of byes. The Ducks beat Arizona 56-31 in their last outing, while the Bears came up short against Washington, 31-23, when the Huskies stopped the final drive on fourth down at the 2.
Cal had allowed just 78 rushing yards per game before playing the Ducks, but the Bears had not played a rushing offense the caliber of Oregon's.
The hoopla surrounding the nationally televised Thursday night game was amplified by Oregon, which wore new "Fighting Ducks" jerseys for the game meant to harken back to the bright green and yellow uniforms of the past.