He'll have turned 28 before the end of the year, he has 1 year left on his contract (albeit with the option to extend for another year), his best ever tally in the league is 7 goals (which he has managed once), and he has never got more than 1 assist in a league season.
I don't know what fair value for him is, but if I was a Fulham fan I certainly don't think I'd be that keen on dropping £30m on him.
And if I was in charge of United, I'd be very aware that we'll only get less for him next summer. Whatever we're being offered for him now, that's his peak remaining value.