The man was taken to Antrim Area Hospital by ambulance and was later transferred to the Royal Victoria Hospital, where he is stable at present.
A PSNi spokesperson said police do not believe at this stage that he was the victim of an assault but would like to rule this out as the cause of his injuries.
Therefore, Ballymena police would like to speak to anyone who may have saw the man, aged in his 30s, in the Castle Street area at the time and can help us establish his movements before suffering the injuries.
Police can be contacted on 0845 600 8000. Alternatively, if someone would prefer to provide information without giving their details, they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555 111.