Hallyday, 66, has been placed in a medically induced coma at the Los Angeles hospital where he is being treated for complications stemming from surgery he underwent in Paris for a slipped disc.
Investigators told the BBC French surgeon Stephane Delajoux was punched and kicked by two men wearing hoods Friday night.
He reportedly was treated for his injuries and released from a hospital Saturday.
The BBC did not suggest a motive for the alleged assault.
Interpol investigating stolen passports on missing flight
Scarlett Johansson steps out with fiance after pregnancy reveal