PFA chief Gordon Taylor has injected himself into the workings of Sunderland under manager Paolo Di Canio.
Two Sunderland players have brought in the PFA to investigate fines imposed by Di Canio last week, says the Daily Mail.
The Italian manager revealed at the weekend that he had fined seven players for breaking club rules in the run-up to their final day defeat at Tottenham.
PFA chairman Gordon Taylor said: "There are rules laid down and we would expect Sunderland to adhere to those the same as every other club.
"I don't want to get into confrontation with them but the players can't be treated just according to what Paolo thinks.
"There are rules and procedures that both he and the club need to adhere to. Yes, the players have come to us for representation. We are aware of the problem. The players did well for him when he came.
"There is a process. If the player has been fined and that is out of the regular code which is described by us and the Premier League, they have the right of appeal. There is a right procedure of doing things that is clearly laid down."
Di Canio has already spoken of his frustration at the PFA becoming involved.
Before the game at Tottenham, he insisted the players had been punished under rules laid down in their contracts.
He said: "Why do they need to speak? They get fined. They are under a private club. The PFA don't pay them, it's a private club that pays them.
"They have to now try and understand because for our player we already received a phone call. It was a clear misconduct."