Liljegren was injured Thursday after he fell into the boards following a trip by Bruins captain Brad Marchand during the first period. Marchand was not penalized on the play.
The 24-year-old Liljegren has one assist and a minus-1 rating in 10 games this season. By being placed on long-term injured reserve, Liljegren will be sidelined a minimum of 10 games and 24 days.
He has 43 points (11 goals, 32 assists) and a plus-34 rating in 151 career games since being selected by the Maple Leafs with the 17th overall pick of the 2017 NHL draft.
Also on Saturday, the Leafs recalled defensemen Simon Benoit and Max Lajoie from the Toronto Marlies of the American Hockey League.