The team announced Friday that the veteran linebacker will miss at least a month of action with a broken jaw. Matthews suffered the injury against the Seahawks on Thursday.
His recovery timeline would have him missing the next three games and be possibly available to return against the Steelers in Week 10. Los Angeles has its bye week in Week 9.
Matthews, 33, has started all five games and accumulated six sacks, two forced fumbles and 19 total tackles in his first year as a member of the Rams.