LAS VEGAS (AP) — Justin Fields hyperextended his left leg and got the wind knocked out of him on a tackle in the second quarter. Although Chicago’s rookie quarterback was seriously shaken up, he walked to the sideline and only stayed there for two plays before barging back onto the field.

Fields is determined to be a dependable leader for the Bears, even when their defense is leading the way to another victory.

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Fields threw his first career touchdown pass, and the Chicago defense largely shut down the Las Vegas Raiders’ high-powered offense in a 20-9 win Sunday.

Damien Williams scored a touchdown and combined with Khalil Herbert for 139 yards rushing in the Vegas debut for the Bears (3-2), who were accompanied to Sin City by thousands of vacationing fans for the latest meeting of two beloved NFL franchises that failed to win a playoff game in the previous decade.

Jesper Horsted caught the first TD pass by Fields, who went 12 of 20 for 111 yards in the touted Ohio State product’s third career start — his first since being named the Bears’ No. 1 quarterback by coach Matt Nagy.

Fields wasn’t satisfied with the offense’s quiet performance, but he was determined to finish what became a 16-play, 86-yard scoring drive despite the scary tackle.

“I was just trying to see if I could walk off, and after a while I could feel myself getting my strength back,” Fields said. “I’m always going to bring that to the table. I’m going to put myself out there to win games, and I’m going to give it my all.”

The Bears’ 32nd-ranked offense managed just 252 yards, but Chicago’s defense allowed three points from Vegas’ first seven drives. Khalil Mack led the Bears’ dominance of his former team with eight tackles and a sack of Derek Carr, his good friend, on a 2-point conversion attempt in the fourth quarter.

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“This was going to be a fight, and he knew that,” Nagy said. “Khalil is in a leadership position to show the other guys how to play in this moment, and he did that today.”

The Bears got ample help from the Raiders’ untimely first-half penalties — including a holding call on Foster Moreau while he attempted to block Mack that negated a touchdown by Josh Jacobs.

Carr passed for 206 yards and Jacobs rushed for a late touchdown in the second straight loss for the Raiders (3-2) — their first with fans in the stands at 2-year-old Allegiant Stadium. Vegas was penalized 10 times for 82 yards.

“Way too many penalties, I think,” coach Jon Gruden said. “When you have eight holding penalties and you struggle in a short-yardage situation and field position is hard to come by, it’s going to be a fistfight, and they won this one today.”

Las Vegas struggled on offense in Gruden’s first game since the revelation of the veteran Raiders coach’s racist comment about players’ union leader DeMaurice Smith in an email in 2011. The NFL is looking into the matter, and Gruden could face league discipline for comments made back when he was employed by ESPN.

The Raiders uniformly rejected the idea that Gruden’s troubles had been a distraction. Gruden, who reiterated his belief that he isn’t a racist, addressed the controversy at a recent team meeting.

“I can understand the hurt that (Smith) is probably feeling, so it’s hard,” Carr said. “I love (Smith) and I love Coach, and I’m just trying hard to navigate through that.”

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Cairo Santos hit two 46-yard field goals and Chicago’s defense stopped Vegas on downs in the final three minutes.