Despite only five wins in his last 30 league games, anyone who questions the Sunderland manager's ability is 'absolutely ridiculous', according to the Manchester United boss
The Manchester United boss launched a defence of the former Aston Villa manager, who was asked whether he still had self-belief earlier this week.
Sir Alex believes such questions are an insult to a manager who has acheived what O'Neill has in his career.
"Martin O'Neill has won three titles with Celtic and taken them to the Uefa Cup final," he told BBC Sport.
"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."United memorably lost the Premier League title on the final day of last season despite a 1-0 win at the Stadium of Light. Sir Alex described the reaction of the Black Cats' fans as 'disappointing', as they celebrated United's misery.
"It was a disappointing reaction when you think of the number of players we've given [loaned] Sunderland over the years," said the Scot.
"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."