Guys, I happen to be Uruguayan, and I work in football. Now I live in Europe, still working in football.
I wanted to explain something most of you guys may not know. In spanish, saying "negro" is not an insult. Not at all. In fact, in Uruguay it is a colloquial term that is used in the same way the americans use the word "dude". So, "What's up, dude?" is "¿Qué pasa, negro?". It is a usual way to call friends or pals in both Uruguay and Argentina. Many very famous people has "Negro" as their nicknames. And it has nothing to do with being black skinned, let alone with an insult. Just google "Negro Fontova", "Negro Olmedo", "Negro Dolina" or "Negro Alvarez". They are not even black skinned, as most of people nicknamed "negro". Just go and do it, you will see what I'm saying. It is also used as a nice way to call your partner ("Negra" or "Negro"). Hell, *I* use it for sure to talk with other uruguayans or argentinians if I find them on the streets of europe.
So, I have not seeing the action, but I am mostly sure that Luis DID say "negro", because it is a very usual word for us, it is not offensive at all, and I know he uses to speak in spanish when he is in the field. Luis is very used to abuse -in spanish- referees, always. It makes me laugh each and every time I see a Liverpool match, because he really insults so much the referees, sooo much. But never, not even once was he racially offensive to anyone; we mostly concentrate on families when insulting, and that is what he usually does. What we, uruguayans, use to react when angry. Not like we really think the mother of the referee gets paid for sexual encounters with other men. It is just a way to show disagreement in a very strong way, that is all.
In spanish, by the way, the pronunciation of "negro" is with the "e" like the first "e" in "better" and the "o" as in "hostile". It means just "black", and again, in Argentina and Uruguay has the meaning of "dude". Most people nicknamed "Negro" are white skinned.
In my family they call me "Negro". I am as black as Kuyt.