Liverpool's Ryan Babel could face disciplinary action for posting on Twitter a mocked-up picture of referee Howard Webb in a Manchester United shirt after a 1-0 FA Cup loss on Sunday.
Webb, who officiated at the World Cup final last July, awarded United a penalty in the opening minute that Ryan Giggs converted to give the home side a 1-0 win at Old Trafford.
"And they call him one of the best referees? That's a joke," wrote Netherlands international Babel, who did not play in the World Cup final that the Dutch lost 1-0 to Spain in Johannesburg, on the social network.
Babel later apologised after the English Football Association said they would look into the matter.
"My apology if they take my posted pic seriously. This is just an emotional reaction after losing an important game," he added.
New Liverpool manager Kenny Dalglish said after the game he disagreed with Webb's penalty decision and the sending off of captain Steven Gerrard after 32 minutes.