"The football club has commenced a full investigation into the matter," a statement on the official website (www.safc.com) said.
"We expect the highest standard of behaviour from our players and should any individual fall short of those standards then robust action must be taken," said Sunderland's chief executive Martin Bain.
The former Manchester United player joined Sunderland from Everton in January last year but has been injured for the past two months.
(Reporting by Steve Tongue; Editing by Toby Davis)
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