I'm really sorry, I must be being particularly thick today but that confuses me more and I'm not sure if the part about the goalie is true, If a goalie gets sent off then it makes perfect sense to bring on a sub goalie and take off an outfield player but I don't think there's a requirement to do so. And you keep commenting about numbers 4,7, 8 and 10 are these merely player positions quoted in old money, so to speak, and if so, what happens if the team aren't playing 4-4-2?
"You need to take off your centre forward"
"We haven't got one"
I am not 100% sure about this one, but I seem to recall if that numbered player ( 8 or 10 ) isnt still on the pitch due to an earlier substitution, then you must consult the fourth official who in turn would expect that current teams Man of the Match for this game to be sacrificed and asked to leave the field.
Seems a silly rule to me as how do you assess who would be the current best player for that team, but rules are rules.