Actually United can always have Beck, Giggs and Solskjaer as ambassadors. Afterall, they are a big club, and the world is such a big place for one man to be going around to promote them right...
These 3 have their own strong points... Giggs, will probably be the player that played the most matches for united, and that FA cup goal he scored against Arsenal will probably still be the talk of FA Cup matches for the next ten, twenty or maybe more yrs down the road.
Solskjaer will be remembered as the best super-sub united had and the players that scored that historic injury time goal that won them The Treble... He can also double-up as their striker coach (as what he is doing now) when he is not travelling around the world. And Beck... Himself is already the most famous footballer on this planet... So what else does he need to do, Just turn up and meet the Press...
All 3 were part of that Historic Treble winning team...
On another note, Scholes cannot cos he is not a good PR man and he don't like the limelight. Gary Neville also not a very good choice too cos he was never the fans hero type... So that 3 are the best choice i feel...