Chelsea have made a formal complaint after accusing Clattenburg of using 'inappropriate language' during the match at Stamford Bridge which saw the Blues have two men sent off. Mata and John Obi Mikel are reported to have been the subjects of Clattenburg's alleged comments, and Romeu was told of the allegations by his team-mates.
"Neither Juan nor Fernando [Torres] told me they heard it because from what I have understood they didn't hear anything," Romeu, who was an unused substitute, told Spanish radio station Cope.
"It was someone else who heard it, but not directly at them. It was another player who heard it and that is what they told me.
"I didn't hear anything as I left quickly to go home but after talking to Juan he told me there was some problem and he had to stay.
"I only know Chelsea made a complaint and I think there could really be a problem if what Chelsea players say happened really happened.
"If there was really a racist comment or something said against a Spanish player this will be serious.
"We know in this country people are very vigilant about these issues."