It;'s not just about the goals he'll score. It's about the space that he creates by making runs which will allow others (Rashford, Mkhitaryan, Pogba) to break into the centre. There won't be a static Swede and a fat lump in the way. If they find a way to leave Fellaini out of starting line-ups, and cross the ball in from deep for Lukaku to chase, the increased mobility will make them more dangerous. I think it'll easily be worth 10 goals of the pudding.
It does require Mourinho to let more than 3 men attack at a time, but it can't be worse than what they did last year.
Worse, they may not play much of Smalling, Jones and maybe Rojo, which makes the defence better too.
Enough to overhaul 5 teams? Don't know, but I expect them to be a lot closer than last season.