By Dan Durkin-

(CBS) Bears linebacker Lance Briggs has been ruled out for Sunday’s game at Atlanta with a rib injury, the team announced Friday.

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The Bears haven’t revealed the full extent of Briggs’ injury, but for now, it doesn’t sound as if he has broken ribs.

“I don’t know,” coach Marc Trestman said. “He’s sore. That’s really where it’s at, he’s sore. There really hasn’t been any indication of (broken ribs).”

Other Bears who have been ruled out after not practicing Friday are offensive tackle Jermon Bushrod (knee/ankle), safety Ahmad Dixon (hamstring), linebacker Shea McClellin (hand) and center Taylor Boggs (hamstring).

Linebacker Jon Bostic (back) and cornerback Sherrick McManis are (quad, knee) are questionable for Sunday after not participating in Friday’s practice.

Safety Chris Conte (concussion), defensive tackle Jeremiah Ratliff (concussion), center Roberto Garza (ankle) and linebacker D.J. Williams (neck) were full participants and are probable for Sunday.

Uncertainty at the second level of the defense

When the Bears’ base defense lines up at Atlanta, the second level will be without two-thirds of their opening day starters — Briggs at weak-side (Will) linebacker and McClellin at strong-side (Sam) linebacker.

Bostic has replaced McClellin, who has missed the past three games, but he hasn’t practiced all week after suffering a back injury last Sunday.

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Bostic has then been replaced by Christian Jones, who’s also a candidate for playing time at weak-side linebacker and has received more practice time there this week.

“I’ve been working Will since OTAs a little bit and did a lot when we were in camp,” Jones said. “I’m pretty familiar with it. It’s nothing new. But now, they put me in because of the situation with Lance. I’m pretty comfortable with it.”

If Bostic is active for Sunday, it’s possible he will stay at Sam and then Jones or Khaseem Greene — who is listed as Briggs’ backup on the depth chart — would step in at Will.

Connecting the dots here is challenging, given the cascading effect of these key injuries, and the situation remains murky heading into Sunday. Given the Falcons’ preference for 11 personnel, chances are the Bears won’t be in their base defense that often in Atlanta.

Jennings won’t play inside

Another position that’s been affected by injury this season is the slot/nickel corner position. The original grouping in nickel situations was Charles Tillman at right cornerback, Kyle Fuller and left cornerback and Tim Jennings in the slot.

The injury to Tillman sent Fuller and Jennings outside, and Isaiah Frey slid into the slot role. Frey was cut this week, and McManis is still questionable. Trestman mentioned Demontre Hurst and Brock Vereen as candidates to fill the slot cornerback role if McManis can’t play.

“Brock did well, Hurst did well,” Trestman said. “So we’ve got some flexibility there with how we’re going to run the defense this week and there.”

Trestman did rule out the possibility of Jennings moving back inside, saying that “won’t happen this week.”

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Dan Durkin covers the Bears for CBSChicago.com and is a frequent contributor to 670 The Score. Follow him on Twitter at @djdurkin.