The match official is at the centre of a complaint made by Chelsea to the Football Association regarding alleged "inappropriate language" used by the 37-year-old to Mikel during the Blues' 3-2 defeat to United at Stamford Bridge on Sunday.
"I don't believe Mark Clattenburg would make any comments like that," said Ferguson. "I refuse to believe it. There's no way a referee would stoop to that, I'm convinced about that. I think it's unthinkable in the modern climate, I just don't believe it simple as that."
Reports have claimed that Clattenburg, who is currently being investigated by the Metropolitan Police, allegedly referred to Mikel as a "monkey" after showing him a yellow card for dissent during the league clash.