Manchester United manager Sir Alex Ferguson says it is an "insult" for anyone to doubt the abilities of Sunderland manager Martin O'Neill.
The Black Cats boss heads for Old Trafford on Saturday with just five wins from 30 Premier League games.
But Ferguson believes it is "absolutely ridiculous" for anyone to question O'Neill after his previous success.
"Martin O'Neill has won three titles with Celtic and taken them to the Uefa Cup final," he said.
"He's been at Aston Villa, he's been at Leicester City.
"He was asked a question last week which was absolutely ridiculous, about whether he still had self-belief.
"It was a silly question, an absolute insult because he is an outstanding manager."
Meanwhile, Ferguson has described the reaction of Sunderland fans on the final day of last season as "disappointing".
Having beaten the Wearsiders 1-0 at the Stadium of Light, United looked as though they would be champions as they waited on the pitch, only for Sergio Aguero's injury-time goal to earn Manchester City the title.
That prompted cheers from thousands of Sunderland fans.
"It was a disappointing reaction when you think of the number of players we've given (loaned) Sunderland over the years," said Ferguson.
"Jonny Evans and Danny Simpson got them promoted a few years back.
"It's just fans. It wasn't malicious, it was them enjoying the moment.
"Revenge won't come into anything on Saturday.
"Winning the game is more important to us because after the impetus we got from [defeating Manchester City] last week, it's time for us to really kick on."
United start the weekend with a six-point lead, but Ferguson says his side cannot afford to relax.
"It's not such a healthy lead," added Ferguson.
"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."