It's been explained over and over again why people hold that view. It's not difficult to understand that people place more importance on goals scored against 19 teams of similar quality than those scored against lesser opposition in other competitions.
Lukaku scored five goals against Young Boys over two games - yet for me those two yesterday against Southampton have more value than all of those goals combined.
So it works both ways - if he scored 25 in the league and got nothing in the cups, I couldn't care less about the cups, as the league has much more value.
It's the same as a goal from open play being a lot more valuable in assessing quality than kicking in a penalty. One is harder to do than the other. Two away at Southampton is a bigger feat than scoring four against FC Crapo from Kazakhstan.