He reacted by dancing in front of the disconsolate travelling fans at the final whistle Sunday, and television images showed some furious Atletico supporters making monkey gestures.
"It affected me a lot," Diop said. "I went to take a corner and some of the Atletico fans began to make monkey chants. To play it down, I started to dance, but I didn't insult anyone," the player said.
"I don't have anything against the Atletico fans. It is a provocation I get in many grounds. It has to stop. I don't know if it is racism or a lack of respect," he added.
"They have to stop making monkey gestures at black players." A week earlier, Barcelona's Brazilian international Alves shone an uncomfortable spotlight on racism in Spanish football when he took a bite out of a banana thrown at him in his side's win over Villarreal.
Star players of different nationalities and races including Neymar, Sergio Aguero and Luis Suarez joined the "we are all monkeys" campaign.
The culprit at El Madrigal was subsequently arrested and handed a lifetime ban from the stadium by Villarreal.
But Alves said it was not a one-off. "I have been in Spain 11 years and it has been the same for 11 years. You have to laugh at these backward people," he said after the game.
The monkey-chanting at Diop made headlines in the British press, including the BBC and the online editions of the Daily Telegraph and Daily Mail.
"Spanish football must face up to the fact that they have a serious racist problem. They can't continue to bury their heads in the sand," tweeted former England international Gary Lineker who once played for Barcelona.
But the incident created relatively few waves in Spain. "Diop's little dance directed at the Atletico fans was over the top," wrote Barcelona-based sports daily Mundo Deportivo. "But the alleged racist shouts were, too … just like any other insult," it added.
Top-selling sports daily Marca said a delegation of Atletico players visited Diop in the changing rooms after the match to offer their apologies.
The mild reaction in Spain echoed the tone of some comments by Spanish football officials after the Alves banana-throwing scandal.
"It is not everyone. I want to think these are isolated incidents. In football there is no racism, not at all," Spain coach Vicente Del Bosque said at the time.
"You cannot judge all fans by the actions of a few extremists, whether they are from Villareal or Atletico or any other club," conservative daily El Mundo wrote Monday.
Salvador Rodriguez Moya, journalist and author of "Black card to racism" which recounts 300 racist acts in Spanish football, said it would be wrong to condemn all of Spain and all Spanish football as racist, describing the incidents as isolated and anecdotal.
"But it is still true that it is like a sleeping volcano, which could erupt at any moment," Moya said in an interview.
"These racist episodes have happened and continue to happen. We can't avert our eyes," he added.
Ramon Miravitllas, journalist, author and academic who has written about politics in sport, said parts of Spain suffered from ignorance of history and other cultures.
"In deepest Spain there is still today not racism but class structure, selectiveness, exclusion, or a lack of understanding of certain cultures," he said.
"That is the problem, the cultural problem in deepest Spain that does not know this is a country that lived from immigration for many years and that does not recognise it as a multicultural country."