Professional Footballers' Association chief Gordon Taylor says the union is looking into Sunderland's disciplinary measures against several players, reports BBC Sport.
Black Cats boss Paolo Di Canio said last week he had fined seven players and has threatened to get rid of those who have stepped out of line.
Taylor said Di Canio could not be "a law unto himself".
He added: "It's our job to assess the situation and see if it is acceptable or if the players can appeal."
Taylor admitted there had been contact between the PFA and Sunderland players, while discussions had also been held with the Premier League club.
"I wouldn't want to go any further at this moment, so as not to prejudice any particular situation," he said.