Coach Marc Trestman said Friday there was no indication Briggs had broken ribs but was too sore. Briggs hasn't practiced all week.
Briggs' replacement at weakside linebacker would be either Jonathan Bostic, second-year linebacker Khaseem Greene or rookie Christian Jones. Bostic was questionable for Sunday's game with a back injury and also hasn't practiced this week.
Starting safety Chris Conte and defensive tackle Jeremiah Ratliff are both probable for the game after suffering concussions. Starting offensive tackle Jermon Bushrod (knee/ankle) and linebacker Shea McClellin (hand) have been ruled out.