The way is see it is that players that you could get Vidal, Pjanic, Marcheeseio etc etc their clubs know United want players which allows them to bump up the asking some more, then there is the January factor, then there is the fact United could possibly not even make European football next season. So in a way it makes sense that it's difficult.
Oh it does make it more difficult, but will it be easier in the summer if we don't get in the Champions League? We need to offset the risk of that with the extra cost to get the players in now, and really push for it. New signings will lift the rest of the squad too which could be invaluable for the rest of this season, even if it's just one person for now.