The center sustained a Costochondral separation, which occurs when the rib tears away from the cartilage that connects it to the breastbone. He was injured early in the first period of a 5-2 win against the Pittsburgh Penguins on Friday and left the game after playing 45 seconds.
Couturier won the Selke Trophy voted as the best defensive forward in the NHL last season, when he scored 59 points (22 goals, 37 assists) and was plus-21 in 69 games.
He had an assist in a season-opening, 6-3 win against the Penguins on Wednesday.
Morgan Frost skated in Couturier's spot at practice Sunday, playing center between left wing Oskar Lindblom and right wing Travis Konecny. Frost, the No. 27 pick in the 2017 NHL Draft, scored seven points (two goals, five assists) in 20 NHL games last season and hasn't played in a game this season.
"There's no doubt that it's very hard to replace a player like Sean, who plays 5 on 5 against top opponents and he plays power play, plays penalty killing," coach Alain Vigneault said. "But it's next man up and right now it's Morgan."
The Flyers next play Monday when they host the Buffalo Sabres at Wells Fargo Arena (7:30 p.m. ET; NBCSN, NBCSP, NHL.TV).
