Cricketer Phillip Hughes is breathing only with the assistance of medical technology after he was struck by a bouncer that damaged a major artery in the back of his head, broadcaster Alan Jones said on Thursday morning.

Jones, a Sydney Cricket Ground trustee and a friend of the injured 25-year-old, said Thursday was a critical day in the treatment of Hughes, who is in an induced coma after suffering an injury that was "much more serious than anyone had imagined".

"Major artery damage": Phillip Hughes. Credit:Getty Images

A clearly emotional Jones said on his 2GB radio program on Thursday that he had been told that the "neurology is very, very bad, or in the language that the layman understands, the brain is very sick".