The man, who is in his early 60s, was taken to hospital with serious injuries.
At around 4.10 pm on Thursday 16 December, an ambulance attended a flat on Coleshill Road, after a man was found with injuries believed to have been caused by an assault.
The man, who is in his early 60s, was taken to hospital with serious injuries.
A 46-year-old man from Atherstone was arrested and is now on bail while enquiries continue.
Det Insp Gareth Unett, from Nuneaton CID, said: “Although we believe this incident happened inside a property, as part of our ongoing enquiries we would appeal to anyone who may have seen or heard anything suspicious in this area at the stated date and time to please come forward.
“A man is currently in hospital with serious injuries and we need to ascertain how these were sustained.
“We believe the incident may have taken place between late on the Wednesday evening (15 December) through to the Thursday (16 December) morning.
“Anyone with any information, no matter how insignificant, is asked to call Warwickshire Police on 101 quoting incident number 218 of 16 December 2021.”
Aerial photographs of the Bee Hive Car Park taking shape
Man dead after Burbage crash
Free Biker Down Course Offered in Birstall
Serious collision in Quorn leaves two pedestrians injured
Beaumanor Hall chimneys repaired as council looks to protect historic Leicestershire venue
Wildfire Alert from the Government
LCFC supporters group says most support change in ownership
Memorial stall for former market trader
