The £97.5million record signing from Inter Milan has failed to hit the target in his last six games.
Cassano said on Twitch: “Lukaku is struggling, he clings to the defender, and it's his teammates who go looking for him.
“[Thomas] Tuchel doesn't have to learn from [Antonio] Conte how to make Lukaku play; a coach shouldn't dare to say things like that to someone who has won what he has won.
“Tuchel, though, could take all kinds of strikers, but Lukaku has nothing to do with his idea of the game; at PSG, at Dortmund, he had different strikers.
“When you play with Conte 50 metres from the goal, it's easy, and then playing in Italy is another thing.
“In England, you have animals defending; in England, he struggles because Lukaku doesn't have that kind of characteristics.