Good thread.
I presume your point here isn't so much the contribution Lukaku makes, which is excellent, more the lack of contribution from elsewhere.
Only Barkley, and he stuttured for a month or two, has made a siginificant contribution. But Koeman seems to have brought him on over the last few months.
Apart from Barkley our attacking options mostly involve Mirallas, Bolassie, Lookman, Davies ( ? ), Valencia and Calvert-Lewin. Three of them are new to the Premier League this season, one's on loan, one is crocked and Mirallas has always been a bit inconsistent.
I would expect most of Barkley, Lookman, Calvert-Lewin and Davies to improve next season, but, whether Rom stays or goes, I'd say we need someone else in as an attacking option, either an out and out attacker or someone with a bit of guile in their boots.