Sir Alex Ferguson said he was convinced Mark Clattenburg did not use "inappropriate language" against John Obi Mikel, and the Manchester United's comments have not gone down too well with Chelsea manager Roberto Di Matteo.

Chelsea have alleged Clattenburg used language, believed to be of a racial nature, towards John Obi Mikel during the stormy English Premier League clash against Manchester United last Saturday, but according to Ferguson, it is almost impossible for a referee to have done it, considering the current climate.

"I don't believe Mark Clattenburg would make any comments like that," Ferguson told reporters ahead of United's clash with Arsenal at Old Trafford Saturday. "I refuse to believe it. 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."

In reply to Ferguson's remarks, Di Matteo, clearly not amused, said: "He likes to talk about other clubs. We tend not to. I'll leave it up to him.

"It's a free country where everybody has the freedom of speech. We'll take into consideration what other people say and use it as a motivational tool for ourselves."

Asked if he felt his players might have misheard Clattenburg, with Mikel reportedly not even hearing the alleged slur, instead having to wait until reaching the dressing room before being told about it by a teammate, Di Matteo said the investigation would bring out the truth.

"I think the investigation will clarify that," he said. "We cannot comment because we do not want to prejudice any investigation. We will have to wait and see what comes out."

Clattenburg has vehemently denied the charges, which is being investigated by both the FA and the Metropolitan Police, with the referees' union also strongly backing him. Di Matteo, however, is not too concerned about the referees treating his side unfairly on the pitch, in retaliation to the allegation. "I really hope not, and I believe that they won't," he added. "We have faith in the referees.

"They are people that are under a lot of pressure, they are human beings like everyone else and sometimes mistakes can happen."

Chelsea have been embroiled in one controversy after another off late, and the Blues manager believes his side are capable of continuing their business on the pitch, without being distracted by off field issues. "We've been dealing with external situations for quite a period of time and we have been able to focus on our football, so we will just have to carry on like that," the Italian said.