Sir Alex Ferguson does not believe referee Mark Clattenburg used "inappropriate language" towards Chelsea player John Mikel Obi.
The Blues accused Clattenburg of making a racist remark during their 3-2 defeat by United at Stamford Bridge on Sunday.
"I don't believe he would make comments like that," said Ferguson. "I refuse to believe it. It's unthinkable."
Chelsea have made a formal complaint to the Football Association against 37-year-old Clattenburg.
The west London club said the complaint was made following a thorough investigation, which they say was led by outside legal counsel and used information from interviews with Blues players and staff.
Clattenburg maintains his innocence and neither of his assistants, nor the fourth official, support Chelsea's allegations.
Ferguson, speaking ahead of United's Premier League home game against Arsenal, added: "There is not a referee who would stoop to that. I'm convinced by that.
"I've never had a player come to me in the last 15 years and say a referee swore in a game - ever.
"The way we see the game today rather than how it was 25 years ago, it has completely changed. I played myself and I know that the banter which went on between referees and players 25 years ago is different to today."
The Metropolitan Police also started an investigation into the alleged comments after receiving a complaint from a source outside the game.
Chelsea initially suggested Spanish midfielder Juan Mata had also been verbally abused by Clattenburg, before deciding there was insufficient evidence to support these claims.
The Newcastle-based official, who has been stood down for this weekend's round of matches, took charge of his first Football League game aged only 25 in 2000.