Police are investigating an allegation of racist abuse by a spectator during Millwall's match against Birmingham at The Den on Wednesday night.
Birmingham manager Pep Clotet says he reported an incident of abuse aimed at Birmingham player Jeremie Bela during the first half.
Clotet told Sky Sports: "I heard a comment and I reported it to the fourth official, because that is what we get told.
"The protocol activated and now it is in the hands of the authorities. I heard a comment from one individual, not a general thing at all.
"I am pretty sure that these kind of actions should not be heard on a football pitch. All the clubs are working very hard with the authorities to slowly put that behind us.
"We are all humans and the players work very hard to get where they are - and it is our duty to take care of them. But I want to reiterate it was an individual thing and I only heard it once."
Asked about the allegation, Millwall manager Gary Rowett said: "That is assuming that it was heard correctly.
"We had a similar situation recently where an accusation was levelled at the club and it was unfounded. I am not suggesting that is the situation here.
"Up and down the country we don't want to hear the type of comment that has been suggested. I didn't hear any comment, so it is difficult for me to say anything further."
Police say no suspect has yet been identified and that enquiries continue.