Lincolnshire Police are appealing for witnesses of the incident which happened on Monday, January 17.
A man was found unwell on the footpath by the river on Bridge Street by a member of the public just after 2.45pm and taken by ambulance crews to hospital.
He had suffered multiple injuries to the head, face and body.
Police were notified at 11.53pm and they have now launched an investigation into the circumstances and whether the man may have been assaulted.
The police would like to hear from anyone who may have witnessed an assault, or seen a white man in his mid 30s, around 5ft 10 with short dark hair, a beard, and a slim build and carrying a bag on the footpath in the afternoon.
If you can help with the investigation, there are a number of ways to get in touch.
You can call the police using the non emergency number, 101, quoting Incident 424 of January 17, email [email protected] putting incident 424 of January 17, in the subject line, or you can call Crimestoppers, anonymously, on 0800 555 111.
If you’ve been the victim of crime, support is available, whether you have reported the crime to the police or not.
Victim Lincs provide free, confidential advice and guidance, as well as access to specialist emotional support services.
Visit victimlincs.co.uk or call 01522 947510, Monday to Friday between 8am and 4pm.