Lukaku has gotten better and better each season. There is a reason why United and Chelsea are both willing to pay big money for him. The idea that he holds the team back while scoring loads of goals takes a real stretch of the imagination.
All that said though I hope he's a massive flop for United (or Chelsea). I don't think he'll score as many under Mourinho as he did for Everton this year anyway, simply because of the way Mourinho operates, he never really has his strikers scoring loads of goals. If he gets 20 it'll be a very good season for him.