Ronaldo is Juventus’ top scorer this season with 23 goals across 27 appearances in all competitions but he cast an anonymous figure as the Old Lady fell 2-1 to FC Porto in the UEFA Champions League on Wednesday.
The 36-year-old has also failed to score in his last three appearances for the Bianconeri, who have not won in any of those games.
In addition to the midweek loss to Porto, Juventus are playing catch up in Serie A, with leaders Inter Milan eight points clear of the champions.
"I have always said, even though he is a phenomenon and has scored a billion goals, that with Andrea Pirlo's idea of football he could run into difficulties," Cassano said to Christian Vieri on Twitch.
"He scores a goal per game, it's true, but he struggles with Andrea's idea. He's always been a bit selfish, he doesn't give a damn about others scoring goals.
"He's the kind of player that lives to score, not for the game, for the great match. He lives to score and in this moment the situation is getting worse. The years pass for everyone and he is having difficulties.
"The paradox is that Juventus have bought a player who has won five Champions Leagues but has so many difficulties in the Champions League.
"This is because, since [Maurizio] Sarri, Juventus are trying to show a style that is different to its history. It's clear that Pirlo must be judged in four or five years, but Juventus want everything immediately."
