Lukaku has been impressive for Inter since arriving from Manchester United.
He told the New York Times: “He [Ronaldo] told me this is the hardest defensive League in the world.
“He said he has scored goals everywhere, but this was the toughest place to do it, and if Cristiano Ronaldo think it's difficult, then it must be really difficult.
"It's harder than England. The football is more intense there, but here everything is pattern of play."