Originally Posted by
Réiteoir
Van Nistelrooy - definitely onside tonight:
A player leaving the field of play because of momentum during play, is not deemed to have left without the Referee's permission and can therefore re-enter without the Referee's permission. In this instance, although the defender is off the field of play, (and until he returns to the field of play), he should be deemed to be standing on the goal-line (in the field of play) when considering offside. The Assistant Referee should stand in line with the last opponent on the field of play (which in this case will probably be the defending goalkeeper). When deciding offside in this scenario, the two last defending opponents are the defender who has travelled off the field of play, and the defending player who is nearest to the goal line on the field of play (which in all probability will be the goalkeeper).
If an uninjured defender purposefully remains off the field in an attempt to place an attacker in an offside position, then that defender should also be deemed to be standing on the goal-line (on the field of play) when considering offside. Trickery of this nature circumvents the spirit of the offside Law