Nah. It is its own form of fallacious reasoning to scream "correlation is not causation" when the evidence of causation is starring you in the face. Lukaku's absence and our failure to replace him is the biggest factor by far (though, obviously, there are others).
We've not struggled scoring goals. Lukaku wouldn't have scored the 28 more goals than whichever striker has played in his place so far this season to negate the absolute bag of shit we've been in defence.
You can't look at a single game and think 'Lukaku would have scored that' and it would have reversed the result. And thats all he would have brought because he wouldnt have been a better target man, he wouldnt have held the ball up and he wouldnt have worked his arse off like Niasse and DCL have done.
But you can look at games where options instead of our CBs or LBs could have changed the result for the better.