Correct.
Unless we do something completely unexpected in the market before the season starts, what it truly comes down to is players from the 2016-17 squad improving their overall game and - yes, crucially - chipping in with more goals.
Lukaku is not Zlatan, though. Which works both ways, you could say. In terms of goal scoring, as such, he isn't a player who will score goals at the detriment of the team performance - to put it in somewhat extreme terms. If he scores 30+ it will likely be as a result of our overall attacking game improving compared to last season. At least I hope so.
In other words, a high total for Lukaku shouldn't rule out what we need from Pogba, Mickey, Rashford, Mata, Martial (I still hope there's a place for him as an important player for us) et al.
It's obviously true that we frequently achieved great things under Fergie without having a grand goal machine in the team (certainly not in post Messi/Ronaldo terms), but for various reasons a direct comparison with those seasons is simply not relevant.