Hedman was visibly frustrated as he left the ice.
Hedman has been a Norris Trophy finalist for four straight years, winning the award in 2018. He posted 55 points in 66 contests while averaging over 24 minutes per game this season.
"We've got to have that next-man-up mentality until he gets back," McDonagh added.
The Lightning were already without captain Steven Stamkos, who's yet to appear in the postseason due to an undisclosed ailment.