Mark Clattenburg could not have directed a racial insult at a player, said Manchester United manager Sir Alex Ferguson Friday.
"I don't believe Mark Clattenburg would make any comments like that. I refuse to believe it," Ferguson said. "I think it is unthinkable in the modern climate. I just don't believe it -- simple as that. There is no way a referee would stoop to that, I am convinced of that."
Chelsea manager Roberto Di Matteo said negative comments from Ferguson were nothing new. Chelsea filed the complaint against Clattenburg on Monday.
"It is a free country and everybody can say what they want," Di Matteo said. "He likes to talk about other clubs. We tend not to. I'll leave it up to him. It doesn't really affect me too much to be honest. We take into consideration what other people say and use it as a motivational tool for ourselves."
Police and the Football Association are investigating allegations from Chelsea that Clattenburg used "inappropriate language" toward John Obi Mikel in last weekend's stormy clash against United at Stamford Bridge.
Chelsea submitted a file of evidence to the FA containing accounts from players, who allege Clattenburg used a term which has been interpreted as racist. Ferguson has been no stranger to run-ins with referees during his near 26-year reign at Old Trafford but he said he believes in the integrity of the officials.
"I think in the modern game, the way we see the game today rather than how it was 25 years ago, it has completely changed," he said.
"I played myself and I know the banter that went on between referees and players 25 years ago is different from today.
"I have never heard a player come to me in the last 15 years and say a referee has sworn at them during a game, ever. So, that is where I stand. I don't believe it."
Clattenburg has received the full backing of his fellow officials during any investigations. Assistant referees Michael McDonough and Simon Long, and fourth official Michael Jones said they did not hear anything untoward said on their linked headsets.
Information from Press Association was used in this report.