Sir Alex Ferguson is undeniably one of the greatest managers in football history.

During his 27 year tenure at Manchester United, the Scottish manager guided the Red Devils to 13 Premier League titles, two Champions League wins and five FA Cups.

He is also credited as the manager who helped the Red Devils re-establish themselves as one of English football's most successful sides, ending the club's 26-year wait for a top-flight trophy after winning the inaugural Premier League title in 1993.

The foundation of his success was the incredible team he built around the group of young players known as the "Class of 92", who would go on to win countless Premier League titles and European honours.

Ferguson, meanwhile, amassed one of the most respected managerial records in the history of the English top-flight, taking charge of 810 games at the club while winning 528 of those games.

It's safe to say the 80-year-old knows what he's talking about when it comes to football.

But as many, including Roy Keane, found out, he wasn't the type of manager you'd want to cross and especially didn't like it if you said his team were underperforming.

MANCHESTER, ENGLAND - MAY 26: Sir Alex Ferguson of Manchester United leaves the pitch during the Manchester United '99 Legends v FC Bayern Legends at Old Trafford on May 26, 2019 in Manchester, England. (Photo by Nathan Stirk/Getty Images)

Back in 2011, Manchester United crashed out of the Champions League at the group stages after struggling to defeat Benfica and Basel.

While Ferguson's side had been beaten by Barcelona in the final of the 2010/11 tournament, the hangover from that tough loss seemed to linger over his side during the following campaign.

And as his frustrations boiled over at his side's poor performances, one journalist bravely questioned him in a press conference following his sides 2-2 draw at home to Benfica.

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The reporter asked: "Given that the two best teams in the Premier League [Manchester United and Manchester City] are struggling in Europe this season, are standards slipping in the Premier League or is this one of those things?"

Looking puzzled at the journalist's question, Ferguson responds: "Say that again?"

The journalist obliged, saying: "The two best teams in the Premier League are struggling right now in Europe-"

BLACKBURN, ENGLAND - AUGUST 28: Sir Alex Ferguson, the Manchester United manager, tells the referee to check his watch during the FA Barclays Premiership match between Blackburn Rovers and Manchester United at Ewood Park on August 28, 2004 in Blackburn, England. (Photo by Gary M. Prior/Getty Images)

But was cut off by Fergie, who incensed at his comments, snapped: "Struggling? Thank you.

"Are you serious? We're not struggling," he responded before getting up and walking out of the room.

Take a look at the video below...

An awkward moment for all involved, but Ferguson was clearly irritated by the journalist's comments.

It would turn out to be an even more frustrating year for Ferguson and Manchester United, who would miss out on winning back-to-back league titles after their rivals Manchester City would beat them on the final day of the season, thanks to Sergio Aguero's infamous last-gasp winner.

While it was a season to forget for Ferguson, it does little to tarnish the incredible things he accomplished during his time at Manchester United.