It's not more difficult at all, that's a nonsensical argument. Utd aren't going to sign a player for a competition they don't even care about and we aren't going to deliberately weaken our team in a competition we DO care about just because we want to make Utd happy.
He's our player and we don't need to sell him. If Utd want him and he is cup tied, tough sh*t. Either pay up and live with it (they won't, he's worth more to us than them) or go somewhere else.