Barton, 26, had previously admitted the charge of violent conduct and was fined £25,000 at the personal hearing.
He will be banned for another six games if he is found guilty in the future.
In July Barton received a four-month suspended prison sentence for the assault in May 2007, when the pair were team-mates at Manchester City.
The disciplinary panel revealed its findings at a the three-hour hearing at the Football Association's headquarters in Soho Square, London.
Former Newcastle boss Kevin Keegan, who quit on Thursday, faxed his support but did not appear in person as a character witness for Barton.