PEP Clotet confirmed the comment he reported to officials at The Den on Wednesday night was of an alleged racial nature which he said was aimed at Jeremie Bela.
In the first half of the 0-0 draw between Millwall and Birmingham referee David Webb was called over by fourth official John Busby before speaking to Lions boss Gary Rowett.
Clotet explained the alleged comment came from an individual in the crowd: "It's very simple. I heard a comment, a not appropriate comment, that I am forced to report by our rules, the instructions we get from the LMA (League Managers Association) because it's not a comment that should be heard on a football pitch.
"I reported it to the fourth official, the fourth official then followed his protocols. I didn't know the protocols of the referee or the fourth official. I just reported it because I have to report it.
"For my values I have to report it but also for the values of the game and for the values that guide our football here in England. It's a rule as well. I must report.
"Let me qualify one thing. It was one individual. Unfortunately these comments happen in football stadiums, it has nothing to do with Millwall, it has nothing to do with a club. It is one particular individual. A comment that was not appropriate and was reported. Now it's a matter for the authorities.
Asked if the alleged comment was directed towards Bela, Clotet said: "Yes. My personal opinion is that unfortunately things like this happen in all of the football clubs, or some of the football clubs or whatever. And I know all the football clubs and the leagues and authorities are working very hard to move these things away.
"At the end of the day we are all equal and it doesn't matter where you're from, what your religion is or sex is. We need to accept differences and differences makes us better.
"It's nothing related to a football club, it's just individuals who are in the wrong place shouting the wrong thing.
"It goes into the hands of the referee and then the authorities.
"I don't like when someone goes, 'we've been at such and such a club, and such and such a club is this'. No. it's one individual, it has no effect on the club, you know what I mean? We all have to work together to move this out of our game. Simple as. This has no place here."
Rowett spoke in the post-match press conference before Clotet.
Rowett said: "I think Pep has heard a comment, an abusive comment, I'm not sure whether it was to one of his players or staff, or whatever it was.
"Obviously he's then reported it to the officials which is what we're supposed to do, or told to do. I hadn't heard the comment or what was said, or what was alleged to be said.
"My stance on it is really simple. If there has been an abusive comment, I don't think there is any place in any football stadium or anywhere for that matter, to condone that type of behaviour.
"I think it's something the club will have to investigate and if it's proven to be the case then certainly action will be taken."
Image: Millwall FC