Sunderland have started an internal disciplinary process against Darron Gibson after footage emerged of the midfielder criticising teammates in a bar.
The Republic of Ireland international was recorded in conversation with fans following a 5-0 friendly defeat to Celtic and offered some frank and at times foul-mouthed verdicts on the commitment levels of some in the dressing room and the overall state of the club.
The Black Cats said in a statement: "Darron Gibson has not conducted himself in a manner befitting Sunderland Football Club. As a consequence, we will initiate our internal disciplinary process to deal with the matter.
"Darron has apologised this morning."
Saturday's thrashing by the Scottish champions proved a bitter way to mark the 20th anniversary of the Stadium of Light.
Gibson was a second-half substitute in the match, arriving with the scoreline already at 5-0, and was filmed debating with supporters, seemingly under the influence of alcohol.
Gibson made 12 appearances for the Black Cats after signing from Everton in January.