SANTA CLARA (AP) — As a fourth-round pick with nothing more than a fill-in performance on his NFL resume, San Francisco 49ers rookie Dontae Johnson knows what to expect when Jay Cutler and the Chicago Bears see him matched up against a wide receiver Sunday night.

“That’s one of those things they say, ‘He’s a rookie. Let’s see if he can play this game,’” Johnson said. “But that’s one of those things that comes with being a rookie. You got to be able to step up to the challenge, step up to the plate and make plays when they’re presented to you.”

Johnson passed his first test last week.

Starting cornerbacks Tramaine Brock and Chris Culliver and rookie nickel cornerback Jimmie Ward left San Francisco’s 28-17 season-opening win at Dallas with injuries. Brock injured a toe, Culliver sustained a concussion and Ward had a head injury.

Johnson and journeyman Perrish Cox entered in relief against Tony Romo and the Cowboys’ prolific passing game. The pair smothered Dez Bryant, Terrance Williams and the rest of Dallas’ wide receivers, playing with smarts and savvy as if they had done it together so many times before.

“It kind of caught me off guard, too,” 49ers linebacker Patrick Willis said. “I wasn’t sure because we have different packages. I saw P-Cox come in at certain times I was kind of confused for a second. And then I saw the youngster out there and I was kind of like, ‘What’s going on?’

“And then I realized both our starting corners were down. Those guys stepped up and did a great job, and we’re going to have to count on them again this week to do just as good.”

Johnson spent most of last week on the scout team and had been expected to play sparingly on special teams against the Cowboys, while Cox has proven to be a valuable replacement the past few seasons.

Johnson said he’s approaching this week the way he has all offseason — as if he will see significant time on defense. He was drafted 129th overall out of North Carolina State, where he played both cornerback and safety positions at various points, and his versatility has shown right away with San Francisco.

Johnson had a team-high two pass-breakups in the win at Dallas, and one of the few defenders to outshine his performance was his fellow secondary substitute.

Cox did more than just play the role of a replacement. He had one of San Francisco’s three interceptions against Romo as the defense dominated the day.

“You don’t have to teach puppies how to bite,” 49ers coach Jim Harbaugh said. “Don’t need any training to do that — they know how. I was excited for them, excited for the way the guys played. They did a great job.”

The Bears could present San Francisco’s cornerbacks with an even bigger challenge this week.

Brandon Marshall, Matt Forte, Alshon Jeffery, Martellus Bennett and Santonio Holmes headline one of the league’s deepest and most talented receiving groups. And coming off a disappointing opening loss to Buffalo, they don’t lack for motivation.

Defensive coordinator Vic Fangio said he wasn’t surprised by his cornerbacks’ performances in Week 1, though he admitted rookies such as Johnson don’t always perform so well early in their career. Fangio said Johnson “matched” the expectations the team had for him, but he’s far from reaching his potential.

“I think he was going to be capable. I don’t know if it was going to be that Sunday,” Fangio said. “I do think he’s capable emotionally and mentally to do it. Now being asked to do it on opening day of his rookie year may be a bit of a stretch, but he went in there and did very well. Obviously, pleased with that. Hopefully, he grows with that and does not rest on his success.”

NOTES: Fangio said DT Ray McDonald has remained focused in practice and in meetings since being arrested on suspicion of domestic violence Aug. 31. McDonald has not been charged. “I think he’s handled it extremely well in light of the firestorm that’s out there,” Fangio said. ... Brock and RT Anthony Davis (hamstring) did not practice, while Culliver was in a blue non-contact jersey.