The 53-year-old victim was discovered at Premier Drive off the Shore Road around 2.30 am on Saturday by a passerby.
Police know the man’s identity but are trying to find out how and why he suffered a head injury.
He is currently in hospital.
Detective Sergeant Melanie McGrory said: “We believe the man left a bar in the Shore Road area at about 1.20 am and we are keen to trace his movements prior to being found in Premier Drive around an hour later.
“Anyone who may be able to assist police or who saw a man with dark hair, heavy build and wearing dark trousers and a dark top, in Premier Drive or any of the surrounding streets during the early hours of Saturday, should contact detectives at Belfast Reactive and Organised Crime by calling 101.
“Or, if someone would prefer to provide information without giving their name, they can contact the Crimestoppers charity anonymously on 0800 555 111.”