Juventus defender Cannavaro suggested the 18-year-old might have to leave Inter in January to land a place in the Azzurri's World Cup squad next year if he is not playing regularly for the Serie A champions.
Santon made his Italy debut in June after some impressive performances for Inter in the second half of last season, but Mourinho did not use him in their first two league games this term.
"Players like him and (Inter's 19-year-old striker) Mario Balotelli must play and not in training," Cannavaro told reporters at a training camp for Italy's World Cup qualifiers in Georgia on Saturday and at home to Bulgaria on Wednesday.
"He must find a way to play," he added when asked what Santon should do if he is still not getting any action in January. "Otherwise, he risks missing the World Cup."
Mourinho hit back by warning Cannavaro his Juventus team mates might be the ones who end up taking his advice.
"Cannavaro is still a player but he talks like a coach, director of sport or president," Mourinho told Inter's website. "But he is mistaken and he forgets some details that do not concern Inter.
"For example, given that he and (Giorgio) Chiellini play at Juve, (Nicola) Legrottaglie must evaluate whether to change team in January or he'll miss the World Cup.
"And young talent (Sebastian) Giovinco must assess his options for the future well too."
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