For a start they would have to make him the highest paid footballer in the world. Then they would need to raise Rooney's wages to match him as his contract demands.
Basically it would cost Man Utd upwards of £100m just for the contract, and about the same again in transfer fee.
On that basis I would love it to happen, but it just won't.
Not lost on me how they are OK with this kind of spending, when it's them and not City or Chelsea

Exactly why they were so happy about PL FFP rules and in fact United worked hard to get them introduced. That's how they combated City
Remember Jose primarily got sacked because of bitching over no money for signings, after what he spent ? RA wasn't about to tolerate dire football and whinging for another half a billion
is when their fans criticise City for spending and FFP, which has kicked off again since City caught them up in the league this season 
