Law 5 from the IFAB Laws of the Game establishes the basic principle that, if a soccer referee stops play because a player had been judged to be seriously injured, that player is required to leave the field for treatment and may not return to the field (assuming the player is not substituted) until after play has been restarted and only with the permission of the referee. In 2012, US Soccer Federation's guidance on this matter, based on interpretations provided by the International Board, establishes the following additional information:
- The goalkeeper is exempt from this requirement.
- A player who is injured may still be required to leave the field if the referee beckons medical assistance onto the field at a stoppage called for some other purpose.
- A player who leaves the field -- with the permission of the referee, for treatment of an injury without having assistance beckoned on or for whose injury play has not been stopped -- may return before play is restarted, provided the player receives the permission of the referee.