The English Football Association has launched an investigation into an official complaint from Chelsea after the club accused referee Mark Clattenburg of verbally abusing two of their players in their defeat to Manchester United.
Chelsea says Clattenburg used 'inappropriate language' towards two players.
The club would not name the players in question or of what nature the remarks were, but a spokesman refused to deny that Nigerian midfielder John Obi Mikel had visited the referee's room after the game.
Mikel was booked for dissent during the second half.
Clattenberg also sent off Chelsea players Branislav Ivanovic and Fernando Torres in the game.
Chelsea host United again in Wednesday's League Cup fourth-round match and although teams often rest leading players in the competition, the clash may take on extra significance given the controversy surrounding Sunday's Premier League encounter.