If a player suffers what are deemed as “mandatory signs of concussion” (i.e. loss of consciousness, lying motionless for more than five seconds, confusion/disorientation amnesia, vacant look, incoordination, tonic posturing, impact seizure or ataxia), they must be removed from the field of play.
While some sports organisations, e.g. the NFL, demand a player is removed once they’ve shown a mandatory sign, this is not the case in FIFA-sanctioned competition.
However, the FA has instructed its members that if there is a loss of consciousness on the field of play, a player must be removed and not allowed to return. Video replays can be used to clarify events, if available.
If there is any suspicion at all of a concussion, players are supposed to leave the pitch and not return. Furthermore, if they are deemed fit to go home, they are not allowed to drive or be left alone in case of a deterioration of their condition.