The call was for elbowing, but it's not clear if he actually hit him with the elbow or if he simply followed through with the elbow after the hit. But either way, three things are true here: 1) He hit him in the head, 2) It was a late hit and 3) It was a dumb hit given the situation.
When your team is down two goals in the third period of a playoff game with a chance to clinch a spot in the Stanley Cup Final, that's not really the time to be putting your team on the penalty kill for five minutes.
Montreal's Brandon Prust was suspended two games earlier in the series for a similar hit on Rangers forward Derek Stepan. That hit left Stepan with a broken jaw and kept him out of the lineup for Game 4 of the series.