Just to add to that - here's last seasons goal totals:
Tottenham - 86
Chelsea - 85
Man. City - 80
Liverpool - 78
Arsenal - 77
United - 54
United has to find 25 more goals, minimum. And it's not like that was a one off problem ... 15/16 United scored 49 goals. In 14/15 it was 62. In 13/14 it was 64. The reality is that the attackers currently at the club don't score goals, and haven't scored them in the past either. It's the worst attack of the top 6, and by a pretty substantial margin. Replacing Ibrahimovic with Lukaku improves it a little bit maybe, but United needs a lot more than a little bit. They need a massive improvement, and I just don't see where it's magically going to come from.
It's a title contending team in every area but the attack. They just won't score enough goals to keep up. I'll give them, generously, 70 goals this season. That gets them 3rd/4th with a stingy defense, but no higher.