Rielly blocked a shot in the first period of Sunday night’s 8-4 loss in Florida, but managed to finish the game.
Teenage blue-liner Rasmus Sandin was called up from the American Hockey League in a corresponding move.
The Leafs said Rielly, who underwent a CT scan Monday afternoon, would be placed on injured reserve and reassessed after eight weeks.
The 25-year-old has recorded 27 points (three goals, 24 assists) in 46 games this season. The Vancouver native has 270 career points (54 goals, 216 assists) in 516 regular-season games and 15 points (two goals, 13 assists) in 20 playoffs contests.
The Leafs have also been without fellow top-4 defenceman Jake Muzzin since he broke his foot on Dec. 27. The team said at the time 30-year-old was week-to-week, but he was placed on IR before Sunday’s loss.
Based on the injury timeline, Rielly will miss at least 23 games, with a potential return date set for March 10.
Sandin, 19, made his NHL debut Oct. 2 and registered two assists in six games with the Leafs before getting sent down.
In 21 contests with the AHL’s Toronto Marlies this season, Sandin has two goals and 15 points, including three assists over the weekend.
The 29th pick at the 2018 NHL draft led all blue-liners with three goals and 10 points in seven games at the recent world junior hockey championship to help Sweden win bronze.
Sandin was named the under-20 tournament’s top defenceman and also received an all-star nod in the Czech Republic.
The Leafs, who have given up 17 goals in their last three outings, open a three-game homestand Tuesday against the New Jersey Devils.