Police received a report a man had been found collapsed on Ormskirk Road, Pemberton, near the junction with Ellesmere Road, at around 3.25pm on Tuesday.
The 28-year-old man had been found unconscious by a passer-by.
He was treated for a head injury by paramedics and taken to hospital, where he remains in a critical condition.
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Det Con Barry Conway, of Wigan CID, said: "We are trying to establish the circumstances which led up to the injured male being found.
"We are appealing for information from anyone in the area of Ormskirk Road around 3.25pm on Tuesday or anyone with any dashcam or mobile phone footage of the incident."
Anyone with information is asked to call police on 0161 856 3034, quoting 2462-30032021 or the independent charity Crimestoppers, anonymously, on 0800 555 111.
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