Cagliari accused Melo of punching Lopez, causing visible cuts and bruises to his face.
"You all know the basic principles of the Viola approach to sportsmanship," said Fiorentina general manager Sandro Mencucci.
"Any form of violence must be condemned. We know there was a dispute between two players in the tunnel and now the investigation will give its verdict, because nobody really knows what happened.
"Other players intervened to split them up. I have not spoken to Melo yet, but we need to hear from everyone and evaluate the situation. We have taken action against our players for far less."