Speculation over O'Neill's future as Black Cats boss has rumbled on for most of the season as the club's fortunes have nosedived.
Sunderland were in the Barclays Premier League's bottom three until they beat fellow strugglers Reading, only their third victory of the entire campaign, and few expect them to record a first win over United at Old Trafford since 1968 when the two sides meet on Saturday.
But Ferguson does not believe such struggles should lead to talk about O'Neill's qualities. For the United boss, O'Neill's CV speaks for itself.
"He was asked a question last week which was absolutely ridiculous, about whether he still had self-belief," said Ferguson.
"Martin O'Neill has won three titles with Celtic, he's taken them to a UEFA Cup final, he's been at Aston Villa, he's been at Leicester City. It was a silly question, an absolute insult because he is an outstanding manager."
Not that Ferguson was too impressed with Sunderland supporters on the last day of last season.
Having beaten the Wearsiders, United looked as though they would be champions as they waited on the pitch, only for Sergio Aguero's injury-time goal to snatch the title away for Manchester City.
Gloating from the home fans was immediate and Ferguson was visibly irritated at the time.
Wayne Rooney followed up in his autobiography by claiming he would look for the Sunderland fixtures with added interest this term.
However, time seems to have brought a measure of perspective to the situation.
Ferguson still cannot understand the reaction, but all he is interested in now is preserving his side's six-point lead at the top of the table.
"It was a disappointing reaction when you think of the number of players we've given Sunderland over the years. Jonny Evans and Danny Simpson got them promoted a few years back," he added.
"It's just fans. It wasn't malicious, it was them enjoying the moment.
"Revenge won't come into anything tomorrow [Saturday]. Winning the game is more important to us because after the impetus we got from last week, it's time for us to really kick on."
Ferguson admitted the manner in which that day unfolded hurt badly though and he has no wish to experience it again.
United have certainly made a decent start, their win over City opening a handy lead on their major title rivals.
However, after losing an eight-point advantage in their final six games last term, Ferguson is taking nothing for granted.
"I am happy with six points, it's their challenge to try and catch us," he said.
"But it's not such a healthy lead.
"The key is whether we can keep all our players fit.
"If we do that, with the squad we have, I will be really happy.
"First of all we've got to get through this heavy Christmas programme.
"If we are still top of the league when we've navigated that we will be delighted."
Ferguson has a major boost ahead of Saturday's game as skipper Nemanja Vidic is available for the first time in three months after a second knee operation.
It has not been decided whether Vidic will start, although Ferguson does have a vacancy now that Jonny Evans has been sidelined by a hamstring injury.
Ferguson would hope to have the Northern Irishman available at some point over the festive programme.
Japan star Shinji Kagawa should definitely be available, after overcoming a knee injury suffered against Braga in October.
The 23-year-old is due back in full training on Monday though.
"It is really good news," said Ferguson. "He is doing very well now."