Del Bosque believes the power of Lukaku is almost uncontrollable.
He told Gazzetta dello Sport: "He is one with whom defenders are always uncomfortable.
"There is always the debate about who bothers the opponent the most, the physically strong centre-forward, the tank, or a faster and more skilled one: well, I think that today for a defender there is no more complex person to mark and stop than Lukaku. And then there are no players suitable for one or another era. If one is strong he is always strong. And if one is mediocre, maybe he can improve somewhere, but in the long run he remains mediocre.
"Lukaku is only 27 years old but seems to have played for a lifetime, and has done well in the past. But I have the impression that Inter have found the ideal environment for him, both on the pitch and on a more general level. Lukaku's strength is that he doesn't think about exams whatsoever, it seems to me that everything bounces on him, on and off the field. I have the impression that he is completely indifferent to circumstances, whether they mark him as a man or an area, whether they defend themselves in 10 or leave space.
"He lives in his own life."