Symptoms sometimes do take time to appear, but in this case he clearly to a heavy blow to the head and was visibly dazed on the field.
There's a reason the NHL has the quiet room and the NFL has the concussion tent, it's to give those symptoms time to appear.
I'm not really sure what the solution is since you can't just change the player out for a couple minutes/plays to do a longer review, but something does need to be done because there's currently too much pressure for players to hide small symptoms and stay on the pitch.