Boston leads the series 2-1 but is coming off a 4-2 loss in Game 3 after winning the first two contests at home.
Bergeron was a driving force in Boston's two victories with an assist in Game 1 and a head-turning four assists in his team's convincing 7-3 win in Game 2. The veteran's ability to find open teammates when Toronto crashed on him was critical in the early going of this series, and the Bruins will need others to fill the void while he is out.
Joe Haggerty of NBC Sports Boston called this injury "a devastating blow" for the Bruins.
Bergeron tallied 63 points this season behind 33 assists and 30 goals. The 30 goals tied for the third-highest mark of his career even though he played just 64 games and has been in the NHL since the 2003-04 campaign.
He is also an important part of Boston's defense considering the NHL named him a finalist for the Frank J. Selke Trophy alongside Sean Couturier and Anze Kopitar. The award goes to the forward who is the most skilled on the defensive side of the ice. He is a four-time winner of the trophy.
The only silver lining for the Bruins is the fact the injury was deemed day-to-day, although it was clearly significant enough to keep the veteran off the ice in a crucial postseason matchup against the Atlantic Division rival Maple Leafs.