Cassano went under minor heart surgery in midseason before making a successful comeback.
"After the operation there was optimism, but when I saw athletes like Bovolenta and Morosini suddenly die during games, I wanted to quit," he told Chi magazine.
Olympic medal winning volleyball player Vigor Bovolenta collapsed and died during a match on March 24, aged 37.
"I seemed in shock, incapable of making any autonomous decisions. My wife was so worried, as I just sat on the couch and stared at the wall. I wanted to finish with football.
"I'd say: 'I'm done here. I'm gambling with my life.' But when the medics told me I could go back to playing again, I cried.
"I can never forget the moment I returned to the field. Milan-Fiorentina, the Coach Massimiliano Allegri told me to 'get ready.'
"My legs were trembling as never before, not even when I made my Serie A debut. I was coming back to life and felt like the happiest man in the world."