NORMAN, Okla. (AP) — Oklahoma quarterback Baker Mayfield might have played his way back into the Heisman Trophy conversation.

Baylor quarterback Seth Russell appears to be lost for the season.

Mayfield passed for 300 yards and two touchdowns and ran for another score to help No. 9 Oklahoma defeat No. 25 Baylor 45-24 on Saturday in a game marred by Russell's injury. A season that is fading quickly for Baylor got much worse when Russell severely dislocated his left ankle and was carted off the field with 5:48 left in the third quarter.

"The good news is that I think he'll heal up just fine, but the bad news is that I don't think he'll get back this year," acting Baylor coach Jim Grobe said.

Russell missed much of last season with a broken bone in his neck.

"What he went through last year, to come all the way back here to compete and just getting hurt again — prayers out for him," backup quarterback Zach Smith said.

Baylor (6-3, 3-3) had hoped to bounce back after last week's 62-22 loss to TCU. Instead, the Bears are on their first 3-game losing streak since the middle of the 2012 season. Russell passed for 148 yards and ran for 70 before getting injured, but he threw two interceptions and lost a fumble.

Mayfield got off to a rough start this season in losses to Houston and Ohio State, but he has bounced back and led the Sooners to seven straight wins. Oklahoma coach Bob Stoops said Mayfield is playing better than a year ago, when he finished fourth in the Heisman balloting. He said Mayfield has improved as he has become more comfortable with offensive coordinator Lincoln Riley's system and receiver Dede Westbrook. Mayfield has 26 touchdown passes and just five interceptions in league play.

"Why wouldn't you expect him with another year to be better," Stoops said. "Do we expect? Yes, we do. And watching him from a year ago, having a full year under his belt with coach Riley and these other receivers, and the emergence of Dede — yes, we expect him to be looking like this."

The Sooners won their 15th straight in Big 12 play. Joe Mixon had 187 yards from scrimmage and Samaje Perine ran for 100 yards and two touchdowns after missing three games with a pulled muscle in his right leg for Oklahoma (8-2, 7-0 Big 12, No. 11 CFP).

THE TAKEAWAY

Baylor: The Bears' collapse began with a loss to Texas, got much worse against TCU, and now, things look bleak with Russell out. Smith, a freshman, stepped in to complete 6 of 15 passes for 144 yards and a touchdown Saturday.

Oklahoma: Oklahoma's defense was solid for the third straight week after giving up 59 points and 854 yards against Texas Tech. The Sooners gave up three points against Kansas, then 24 against Iowa State the next game. Baylor gained 504 yards, but Oklahoma's defense forced three critical turnovers.

Linebacker Jordan Evans led the way with two sacks and two interceptions.

"Jordan, he has to play good in the middle of our defense for us to be successful," Oklahoma defensive coordinator Mike Stoops said. "He knows that, and he's really come on when we needed it most."

POLL IMPLICATIONS

Baylor: The Bears' school-record streak of 60 consecutive AP poll appearances, which Baylor says is the fourth-longest active streak in FBS, will likely end.

Oklahoma: The Sooners, who were unranked heading into Big 12 play, could move higher in the AP Top 10 and position themselves to move into the College Football Playoff picture. Oklahoma was 12th in the AP poll at the same point last year.

UP NEXT

Baylor hosts Kansas State next Saturday.

Oklahoma travels to West Virginia next Saturday. The game against the 11th-ranked Mountaineers (No. 16 CFP) provides the Sooners a chance to prove themselves against a high quality opponent.

"Always a challenge," Bob Stoops said. "An exciting challenge. They've always got a raucous, rowdy crowd at West Virginia. It's a challenging place to go play."

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More AP college football: www.collegefootball.ap.org

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Follow Cliff Brunt on Twitter: http://www.twitter.com/CliffBruntAP .