That's just a little bit late, and somehow neither referee on the ice saw it. Or, perhaps one of them did see it and for reasons that remain unclear decided not to call a penalty. At the very least, that should probably be interference.
Losing Stepan would have been a pretty big blow for the Rangers, as he is one of their top forwards. He has three goals and seven assists in the Rangers' first 16 playoff games. He was able to return to the game later in the first period.
The Rangers are already playing without Derick Brassard and also lost Daniel Carcillo to a game misconduct in the first period on Thursday.