The Italy international, who is expected to leave the Premier League champions before Thursday's transfer deadline, was parked outside Indian restaurant Zouk in the city centre on Sunday evening.
One fan was seen defacing the camouflage-painted Bentley, while another dropped his pants before sitting on the back window of the car.
Onlooker Michael Armorgie told The Sun: "I walked past and saw this group of lads. One of them was peeing on the car. I couldn't believe it. One was singing Man United songs."
Meanwhile, Juventus and AC Milan are thought to be in the running to sign the controversial frontman.