Can't really make that out, it's the slightest of shoulder contact if anything. I totally missed that during the game. Whether he was concussed or not he's still not good enough to be our number one keeper.
The thing is he didn't see it coming, so he could not protect himself or prepare better for the impact, Rsmos was moving at speed, so a glancing contact would affect you.
I'm guessing he was spoken to by the physio/doctor, it doesn't appear he was knocked out, as that needs a doctor to clear a player, usually any blackout and the player is taken off.
Where Karius went wrong is jumping back up so fast, he should have kept down and got the doctor on to check him out and get the refs attention to what had happened, I guess this is why VAR is being brought in....