The English FA opened an investigation on Monday into Chelsea's claim that a referee used "inappropriate language'' toward its players during the 3-2 loss to Manchester United.
Chelsea has accused referee Mark Clattenburg of insulting two players in Sunday's match and lodged an official complaint.
"The FA has begun an investigation relating to allegations made following Sunday's fixture,'' English football's governing body said in a statement.
Chelsea midfielder John Obi Mikel was reported to have been one target of Clattenburg's language, which was said to have been interpreted as being racial.
Clattenburg has been backed by referees' union Prospect, which issued a statement that highlighted the wider struggle against racism.
"Prospect is committed to helping to eradicate racism in football and in society generally,'' Prospect national secretary Alan Leighton said.
"In the context of that commitment, Prospect is offering full support to Mark Clattenburg in relation to the allegations made against him.
"It is now important that the allegations are fully investigated through the proper process as quickly as possible.''
Clattenburg will be held out from refereeing for at least the next week due to the investigation.
"Professional Game Match Officials believe that with any football match the focus should not be on the officials but on the players and the game itself,'' the Premier League said.
"Mark Clattenburg is one of the elite referees in world football and, in these circumstances, the intense level of scrutiny would detract from the match and be unfair to the clubs and the supporters of both sides.''
Clattenburg is one of the FIFA candidates to referee at the 2014 World Cup in Brazil.
During Sunday's match, he sent off two Chelsea players - Bransilav Ivanovic and Fernando Torres - and allowed a contentious late winning goal to striker Javier Hernandez, who appeared to be offside when he scored.
The case has plunged English football into renewed controversy just as it was starting to move on from the John Terry racism case.
The Chelsea captain is serving a four-match ban for racially abusing an opponent during a Premier League match last October, a case that has only just been dealt with by an FA disciplinary panel.
Separately, police launched an investigation Monday into a crowd surge that led to a steward at Chelsea being injured.
Chelsea fans rushed forward after Hernandez's disputed winner for United, leading to a male steward injuring a knee and needing treatment at a hospital.
"Police are aware of an incident during yesterday's match at Stamford Bridge in which a steward was injured. An investigation has been launched into the incident and our inquiries continue,'' London's Metropolitan Police said.