This is obviously a big blow to the team's linebackers.
With Stephen Nicholas doubtful and Mike Peterson on injured reserve, the Falcons will turn to some combination of Spencer Adkins and Robert James to man outside linebacker opposite Sean Weatherspoon. While Adkins was fine against the Buccaneers, that's a huge downgrade from a veteran player with a nice combination of athleticism and football smarts.
My best guess is that Adkins starts again and James will be available should an emergency arise. Akeem Dent may be able to swing outside in case of an emergency, as well, but I think the coaching saw enough out of Adkins to roll with him for now.
The best bet for the team. They may even want to try out a 4-2-5 to give themselves more help in the secondary while maintaining a stout front four, because the Giants are a passing team and that needs to be addressed in the gameplan.
How do you think this will affect the Falcons?