Man Utd would barely have any numbers left to use based on this as surely Giggs, Beckham, Scholes and Neville's shirts would be retired as well as perhaps Schmeichel's and a few others!
Imagine if this were their policy Beckham wouldn't have worn 7 and 9 and 10 would have been retired also, along with many, many other numbers. We'd be talking about that great left back No 1452