Who decides that an injured player must come off and be replaced?

A player may be permanently replaced if injured, and may not return once replaced (Law 3.21). Under Law 3.22 a player is treated as injured if:

• at national representative level, a doctor says it would be inadvisable to continue
• in other matches, where the match organiser has given explicit permission, a medically trained person says so; if none is present, the player may be replaced if the referee agrees
• the referee decides, with or without medical advice, that it would be inadvisable to continue, and orders the player off

The referee may also order an injured player off to be medically examined (Law 3.23).

Replacements of any kind come on only when the ball is dead and with the referee's permission (Law 3.6). A player who joins without permission to gain an advantage is guilty of misconduct, and a penalty is awarded (Law 3.7).