2- Pep Guardiola
Yes, I am well aware Guardiola had an agreement with Bayern Munich long before he took charge of the Bavarians.
However I feel that if United had sought him as soon as Sir Alex decided that he wanted to retire, they would have been able to convince him that the Premier League was were he needed to go to enhance his reputation.
An ambitious, young manager with credentials matched by hardly anyone in the modern game, Guardiola took on a talented Barcelona side and made them into one of the greatest sides in history.
His close relationship to his players coupled with his innovative style and pragmatic approach means that the Spaniard has exactly what it takes to succeed at the very highest level.
I hate to refer once more to an earlier point, but his influence over the likes of Thiago Alcantara and the rest of the players he managed at the Nou Camp would have given United a significant transfer market advantage.