The initial point of contact wasn't to the head, but the hit was nearly a full second late, and there was contact to Stepan's jaw that resulted in the Rangers forward suffering a broken jaw, on which he has to undergo minor surgery to repair.
Stepan is questionable for game four of the series, which sounds insane when you consider he requires jaw surgery, but he didn't leave the game on Thursday, which may have impacted the length of Prust's suspension.
Daniel Carcillo ended up taking an even worse suspension earlier this morning in an attempt to get payback on Prust, where he half-heartedly ran Prust from behind and was called for a penalty. Had it ended there Carcillo wouldn't have even been mentioned, but he proceeded to slash, then elbow a linesman, which has resulted in a ten game suspension.
Prust will serve a two game suspension and be eligible to return to the lineup in game six, should the series go that far. Michael Bournival is most likely to be tapped as Prust's replacement for the Canadiens, adding more speed and a better forechecking game to the lineup at the cost of a bit of grit and penalty killing efficiency.