The Sun reports Mellis has been arrested over a bloody fight with his girlfriend.
Mellis, who was axed by Chelsea after a prank with a smoke grenade, was nicked in hospital after being taken to A&E with the 19-year-old following a row at his Surrey apartment.
Police are believed to have kicked in the security door at Mellis' £400,000 home in Cobham as the bust-up in the early hours of Sunday stunned neighbours.
One told The Sun: "All hell broke loose. There was screaming, swearing and the sound of furniture being smashed. It sounded like a full-on war. No one dared go out."
Another neighbour said: "I heard things being smashed and both of them screaming at each other."
Mellis, 21, and his girlfriend were arrested at St Peter's Hospital in Chertsey on suspicion of ABH after being treated for scratches and bruises.
The pair spent most of the day at Staines police station and accepted cautions for common assault.