Spalding CID are investigating a GBH that occurred around 11.30pm on June 11.
The 41-year-old male victim was walking on Ambleside Drive, Spalding, when he was assaulted by another man with a weapon believed to be a baseball bat.
The victim was treated at Boston Pilgrim hospital for head injuries.
Police are appealing for any witnesses to the incident.
DC Matt Nottidge would like to hear from you if you have any information that could assist the enquiry.
There are a number of ways you can report:
• Via the non-emergency number 101, quoting incident number 15 of June 12.
• Through the independent charity Crimestoppers, anonymously, on 0800 555111.
• In an emergency always call 999.