Beckham wore the number seven from the start of the Treble season, if I am not mistaken. That season was arguably his greatest in a United shirt - he made 23 assists and got a fair few goals as well. Wasn't he given Ballon d'Or/Fifa World Player recognition (top 3) after that season? That would suggest that whilst he was established before this, his greatest moment came after receiving the shirt.
Valencia is more established than Ronaldo was when he took the shirt. Sure he was talented, but there was no way of telling that he would become such a star.
When you think about it, what difference does the number make. It is an utter nonsense. The reason that that shirt has had such great wearers is because Manchester United have traditionally had brilliant attacking players. For this very reason, some brilliant players have worn 8, 9, 10, 11. Come to think of it, some great players have worn 1, 2, 3, 4, 5, 6 as well. This numbers rubbish is nonsense.