Briggs (rib), Williams (neck) and McClellin (hand) had been the projected starters, and their injuries leave a huge hole in the Bears' defense against Matt Ryan and the Falcons. Bostic (back) had been one of the projected replacements for Briggs at weakside linebacker.
Only Briggs and McClellin had been listed as out in the final injury report before Sunday.
The Falcons are without receiver Harry Douglas (foot). Two prominent Atlanta players are active who had been listed as questionable - left guard Justin Blalock (back) and receiver-return specialist Devin Hester (hamstring).
Chicago starting left tackle Jermon Bushrod (knee, ankle) also is out.
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