The incident involving a member of staff at The Water Hole in Drummond Road happened on Saturday and is being treated as a GBH assault.
Police are searching for a white male aged 25-40 who is 6ft tall, slightly built, with short dark hair. He was a wearing thin, light blue coloured long-sleeved top, dark trousers, and white shoes/trainers on the evening.
He was with a female with shoulder length blonde hair who was wearing blue jeans and a dark jacket.
A statement from Lincolnshire Police said: “We were called to The Water Hole after reports of an altercation between a doorman and a member of the public.
"During an argument the male punched the doorman to the face, knocking him out and causing GBH-level serious injuries, before running off.”
If any recognises the man in the CCTV image, call 101 or email [email protected], quoting ‘22000319844’.