CARSON, Calif. — Alexander Johnson was ready for his challenge from the start.

Early on during the Broncos' 20-13 win, as the Chargers set up at the line of scrimmage, he noticed quarterback Philip Rivers changed their play. The fullback glanced at Johnson, and with that brief moment of eye contact, Johnson knew he was the target of the play.

"[Rivers] knew I was new," Johnson said. "I knew he checked the run play right at me. And I was like, Come on, number 34 [fullback Derek Watt] — I know you're coming! They sure enough came right at me and I was able to stuff it up and the D-line was able to fall in and Todd [Davis] was able to jump in and make the tackle."

With that, Johnson was full go, playing fast, hard and instinctively as he roamed the center of the field alongside Davis. A week after the Broncos defense gave up 269 rushing yards to Jacksonville, it relied in part on Johnson to prevent an encore performance — and he did his part.

"I'm not surprised by his play," outside linebacker Von Miller said. "He's done it time and time again, and he's a big-time linebacker. He has the size, he has the speed and he's smart. It feels good to have things go his way."

Johnson wasn't just a run-stopper, either. He made two of his most timely and significant contributions defending on two pass plays in the red zone. On the first, he chased down a short pass to stand up running back Austin Ekeler at the 1-yard line, forcing a crucial fourth down at the goal line. The Broncos stood strong and forced a fumble on the ensuing play, saving seven points.

A quarter later, the Chargers were knocking on the door again, looking to cut into the Broncos' 17-0 lead. From the 2-yard line, Rivers took a shotgun snap and immediately looked to the right as tight end Lance Kendricks ran to the corner and Ekeler ran into the flat. Then Rivers quickly looked back toward the middle of the field, where wide receiver Mike Williams was crossing. Rivers unloaded the ball, but he hadn't seen Johnson.

"I was the guy in the middle just reading the quarterback," Johnson said. "I was able to just read him, follow him, watch his eyes. He looked to my left, I thought he was going to throw it, he came scurrying back and looked to the right, so I started scanning back to the right, and the ball was there."

Johnson picked off Rivers to record his first career interception. Without those two plays, the Broncos' 17-0 lead could have been 17-14.

"I think he has the ability to be an NFL linebacker," Head Coach Vic Fangio said. "I thought he played well today, at times. I know he had some flash plays, because even I noticed them from my vantage point. Obviously, the big pick in the end zone, saved three points minimum, maybe seven. He had a chance for another pick. A.J. has good hands, he usually catches that. I'm not surprised he played well. He just has to be a guy that limits the negative plays, too. I'm very happy for him."

Johnson's preparation to start against the Chargers began with a week of practice on the first-team defense. He had to learn a little more, but he embraced his responsibilities, and the results showed on Sunday.

"That was a big thing, getting reps with defense on the first team," Johnson said. "[I was] just preparing myself as usual, as I take any week: watch film, go over plays. I got hit with a whole bunch of new stunts and new calls I had to learn this week. And I had to learn them. Obviously I made a few mistakes out there, but I'm definitely excited to go back and correct myself on those mistakes and get better as a player."

Whether Johnson will start more games likely depends on the status of injured linebacker Josey Jewell, but it's clear that at the very least the Broncos have a reserve they can rely on, someone eager to welcome any trials that he may face.

"I want to take on a challenge," Johnson said. "When stuff's easy, it don't make you better. Anybody can do easy."