Garcia replaced Luciano Spalletti after he stepped down at the end of last season having led Napoli to the Scudetto.
Cassano said on Bobo TV: "Garcia after that gesture by Osimhen has lost credibility. The coach in the locker room doesn't mean s*** anymore, the game is over. One of his players told him in front of the whole world to add two strikers.
"It's not like Kvara who sends him to hell, here we are beyond. Now Garcia either puts Osimhen out of the squad or you no longer count for anything and you're boiled."