Cassano says Lukaku has been struggling with Napoli in recent games.
He said on Viva el Futbol: "(Antonio) Conte's work has been stratospheric so far but he's playing with one player less, Lukaku.
"In fact, it's a big problem, I don't know if he'll ever have what he had at Inter."
Napoli will be leaning on Lukaku in the coming games, with Khvicha Kvaratskhelia suffering a knee strain.
The Azzurri stated yesterday: "During Sunday's match against Lazio, following a clash during play, Khvicha Kvaratskhelia suffered a sprain to his right knee. Today the Azzurri player underwent instrumental tests at the Pineta Grande Hospital, which revealed a low-grade lesion of the medial collateral ligament. Kvaratskhelia has already begun his rehabilitation process."
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