The attack happened at the Royal Oak Bark in Green Street, Carrickfergus, Co Antrim, on Saturday, March 11.
Detective Inspector Lindsay Fisher said: “ Police received a report at around 9.40 pm that a male member of staff at The Royal Oak bar had been assaulted by a number of individuals and sustained serious head injuries.
“He has been taken to hospital for treatment and is said to be in a serious condition.
“Two men aged 36 and 52 have been arrested in connection with the incident and remain in custody assisting officers with their enquiries.
“I would ask anyone who witnessed this incident or who has any information which could assist us with our enquiries to contact detectives in Ballymena on 101 quoting reference number 1275 11/03/17.
“Information can also be given anonymously through the independent charity Crimestoppers on 0800 555 111.”