Nastasic left the Viola in January.
"I will always support Fiorentina in their fight for a Champions League place," he told mozzartsport.com. "They can overcome Milan in the standings, certainly.
"Manchester cannot even be compared to Florence in terms of beauty.
"I will always be a supporter. I appreciate the football they have been playing this year.
"In terms of the quality of their play, there has been no rival this year."