Scenario.... Arsenal v Chelsea, Diego Costa and Gabriel Paulista have been winding each other upfor 80 minutes. Jose Mourinho decides to substitute Costa before he 'does something silly'. As Costa is replaced by Loic Remy, Gabriel says something and Costa returns to the pitch and kicks nine shades of **** out of Gabriel. Now, I know the ref can still give Costa a red card, even though he had left the pitch. But, question is, are Chelsea allowed to continue with eleven players, or does one of their players have to be replaced to leave them with ten men?
He effectively ceases to be a player once substituted therefore would not require reduction of the team.
Incidentally, a manager cannot be shown a red card as he/she is not a player, but can be requested to leave the technical area if disruptive and will be reported for misconduct.