The 39-year-old feels the Partenopei face a tough task in finding a worthy successor and thinks a transfer could be seen by the club's fans as a lack of ambition
The Uruguay international looks set to depart the Serie A club with Ligue 1 champions Paris Saint-Germain leading the race for his signature.
Napoli have been linked with players such as Edin Dzeko and Leandro Damiao in recent weeks but Cannavaro think it will be tough to find a player who is at the same level as the prolific striker.
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Cannavaro, who enjoyed a spell at the Partenopei in the early part of his career, thinks Cavani's departure could frustrate the club's supporters as a transfer of their star man could be seen as a lack of ambition.
"When a player like Cavani leaves, after they have already lost [Ezequiel] Lavezzi last year, it is clear there will be great disappointment," he added.
"These are signals that suggest you do not think about the future. The fans rightly want to win the league and the cup and always hope to achieve the best."
Cavani made 34 Serie A appearances for Napoli last season, contributing 29 goals as they secured a second-place finish in Serie A.