The discovery was made in the Leighinmohr Avenue area of Ballymena.
Detective Sergeant Peter Crothers said: “At approximately 6am, it was reported to police a man, aged in his 30s, was found in a grass area close to a licensed commercial premises, and had serious head injuries.
“The man was subsequently taken to hospital where he remains seriously ill following surgery.
“Police would like to hear from anyone who was socialising in the man’s company earlier that evening at a function he attended in the Leighinmohr Avenue area. In particular, police would like to hear from anyone who was with the man between the hours of 1am and 2:30am.
“Anyone with information is asked to contact detectives in Ballymena on the non-emergency number, 101, quoting reference number 168 08/09/17.
“Alternatively, information can be passed anonymously through the Crimestoppers charity on 0800 555 111.”