#1 Jan Oblak (Atletico Madrid)

Arguably, he is currently the best goalie in the world at the moment. United fans would not be so sad to replace a goalkeeper of De Gea's quality with a man like Oblak.
Great shot stopper, excellent commander of the defence line and able to initiate counter attacks with his long kicks and throws, Oblak's presence will take United's defending several notches higher than it currently is.
His agility would increase the Red Devils' attacking impetus as the midfielders would be encouraged to push up more given the trust they have in his abilities. His confidence, calmness and soft-spoken command of the defence would give the team a solidity it has not had since the Alex Ferguson-era.
Diego Simeone would be loath to lose such an important member of his team. The Slovenian international has won the Ricardo Zamora award for the best goalie in La Liga for the last three seasons. His reported €100m buyout clause would comfortably make him the most expensive goalie in the world.
Should De Gea, it would be a fee that the United board should not hesitate to pay.