Rams head coach Sean McVay told reporters Friday that Matthews sustained a broken jaw after Seahawks running back Chris Carson inadvertently kicked him in the head on a tackle near the end of the game.
Matthews will undergo surgery to repair his jaw, according to McVay. The team didn't provide an exact timetable for his return, but he is expected to be out for at least one month.
"That's not good for us. That's a big injury," McVay said. "He's a tough guy, man, but it was a physical game. It was something that you just hate for him because of how productive he's been for us and what he's meant to our team and all the things that he's done. That's going to be a big loss. Guys are going to have to step up in his absence."
Matthews finished with five total tackles and a sack against the Seahawks. He also drew a roughing the passer penalty in the fourth quarter for a hit on Seattle quarterback Russell Wilson.
Rams outside linebacker Samson Ebukam replaced Matthews and is expected to fill in as a starter in his absence. Ebukam, a third-year pro, started last season and recorded three sacks and three forced fumbles.
Los Angeles originally signed Matthews to a two-year, $9.25 million contract in the off-season. Through five games this season, he has 19 combined tackles, six sacks, two forced fumbles and one pass defensed.