This is the Barca side that he faced.
---------------------Neto---------------------
--------Todibo-Umtiti-Lenglet---------
Wauge-Rakitic-Vidal-Alena--Firpo
-------------Perez-Greizmann------------
It was a B-team.
Napoli are having a poor season. They sit 8th in the table and are in a battle to qualify for the Europa. They're 11 points off the Champions League places.
These are the coefficients
https://www.uefa.com/memberassociations/uefarankings/country/#/yr/2020
The top 7 teams of England, Germany, Italy and Spain compete in these competitions. The good thing about them is that they dispassionately award points to the sides as they progress through the competitions. Whilst some chance factors like the kindness of the draw can affect progress etc its pretty clear that the Italian league is not in the same bracket as the English or Spanish league at the moment.
It is correct that Lukaku can only beat what's in front of him (a bit of a shallow point) and I've always rated him as a good goalscorer but he is doing well against weaker opposition currently. It is what it is. I have zero regrets about him leaving United. In fact I'm glad he left because he's awful to watch whilst being a bitter and small character.