Ristolainen (upper body) was placed on injured reserve Saturday, per the NHL media site.
Ristolainen missed his third straight game against Buffalo on Saturday. He also won't be available for Philadelphia's upcoming three-game road trip, which begins Monday versus Vegas, but it's unclear when he will be ready to return to the lineup. He has three assists, 24 shots on goal, 24 blocked shots and 16 hits in 13 appearances this season.