It’s a big week for the 49ers, coming off a season-opening victory and getting ready for their regular-season debut Sunday night at Levi’s Stadium.

But as the 49ers start work toward beating the Chicago Bears in Game 2, San Francisco suddenly looks vulnerable to Chicago’s passing game.

In the Bears’ Game 1 loss to the Buffalo Bills, quarterback Jay Cutler threw for 349 yards and two touchdowns, and he spread the ball around. Four receivers – Brandon Marshall, Martellus Bennett, Alshon Jeffery and running back Matt Forte – combined for 29 catches. So, there’s no doubt the Bears will put the ball up early and often.

The question is, who will the 49ers have playing cornerback Sunday night?

Starters Tramaine Brock (sprained toe) and Chris Culliver (concussion) both were lost in Game 1, and rookie Jimmie Ward – the No. 1 at nickel corner – also was lost during the victory over the Cowboys to concussion.

Veteran Perrish Cox and rookie Dontae Johnson filled in for Brock and Culliver against Dallas – playing most of the game – and the 49ers could also use veteran Chris Cook (acquired from the Vikings in the offseason) this Sunday night, as well. Cook was inactive for the Dallas game. Chance Casey, a second-year cornerback who was on Oakland’s practice squad last season, also is available on the 49ers’ practice squad.

It appears Brock is the most likely to return for Game 2. He was back on the sideline late in the game against Dallas, and could have played in an emergency. But Culliver and Ward will have to be cleared through the NFL’s concussion protocol. By gametime, it could be a mix-and-match approach by defensive coordinator Vic Fangio to put together a secondary to cover the Bears receivers.

And Marshall and Jeffrey, in particular, are big, fast receivers capable of making game-busting plays. Last year the duo combined for 189 catches, 2,716 yards and 19 TDs.

If Culliver and Ward can’t go, or if Brock’s toe won’t let him play effectively, the 49ers secondary will be severely challenged. Fangio said the injuries during the game against the Cowboys caused the 49ers to adjust.

“It affected the way we called the game, and it affected who was playing,” Fangio told reporters. “We had some packages up where we were going to play six DBs at a time, and obviously that got wiped out kind of early.”

After Sunday’s game, Culliver posted on Twitter that, “I’m OK!! Thank u for the love!!” But it won’t be up to him if he plays. Doctors will want to see progress this week in recovering from the concussion.

If he’s out again, Fangio and the 49ers will have to do a juggling act.