A 45 year old man was discovered with a serious head injury on Tuesday afternoon in Tyldesley town centre.
Andrew Ainsworth, aged 45, of Manchester Road, Tyldesley, was admitted to the Bolton Royal Infirmary at around 2pm after paramedics had been called to Kings Court off Market Street.
He was admitted with a cut to his scalp and a bleed on the brain and remains in hospital.
Police said it is unclear how Andrew sustained his injuries and are appealing for witnesses. They know that shortly before Andrew was found, a dark-coloured car pulled up at Kings Court and someone got out of the car near to where Andrew was found, and returned moments later.
Detective Chief Inspector Howard Millington said: "It is unclear how Andrew has come to sustain what is a very significant head injury. His family are distraught.
"We are keeping an open mind as to how Andrew sustained his injuries.
"I want to speak to the occupants of the dark car that pulled up as they could have vital information.
"I also want to to establish Andrew's movements in the hours leading up to the incident itself."
Anyone with information should call police on 0161 856 7295.