A Costochondral separation occurs when your rib tears away from the cartilage connecting to the breastbone. Symptoms include sharp pain while breathing, coughing, or sneezing.
Couturier took a hit from Jared McCann on Friday during his second shift and didn't return.
Couturier produced 22 goals and 59 points last season and won the Selke Trophy as the league's top defensive forward.
The 28-year-old is Philadelphia's first-line center. It's unclear how the Flyers will fill his spot in the lineup, but moving winger Claude Giroux back to his natural center position is among the team's options.
It's also possible Morgan Frost gets inserted into the lineup. The 2017 first-rounder is a natural center and played 20 NHL games in 2019-20 but has yet to appear this season.