There is no way he can play game 6 if he is diagnosed with a concussion as per the nhl concussion protocol.
Edit: Just to clarify I am in no way condoning what Barzal did at all, definitely a dirty move that does not belong in the game, dude definitely lost his cool. With that being said I think the 5 and a game was sufficient. That is really all I am trying to point out here.
There is no such rule, if a spotter determines a player has a concussion they must sit for the remainder of the game, they aren't forced out of the lineup in future games.