The incident happened between 4.15am and 4.45am at the Subway fast food outlet in Silver Street, on Sunday, April 2. The force has just released details of the attack.
Two men became involved in a verbal altercation which continued outside and resulted in one of the men physically assaulting the other, causing a fractured eye socket.
An 18-year-old man has been arrested in connection with the incident and has been released from custody pending further enquiries.
Anyone with any information about the incident should call Northamptonshire Police on 101 or Crimestoppers, anonymously, on 0800 555111.