Liverpool legend Jamie Carragher has cheekily revealed a promise of what he would do should he ever land the manager's job at Manchester United ... and it involves bringing the club down from the inside.

Sky Sports accidentally captioned video footage of current United boss Louis van Gaal as Carragher instead, which brought about the comedic response on Monday morning.

After a fan tweeted him a screenshot of the gaffe, the retired defender replied, "I'd love that job I could guarantee relegation within 2 years!!"

Putting his pundit head on the day before, Carragher had actually discussed at length the problems he feels United currently have following the shock injury to Luke Shaw last week.

Speaking on Sky Sports' Super Sunday coverage, he explained, "It's given them a massive problem. Valencia and Young played (at full-back) a lot last season but they're wingers. It brings it back to the centre-back situation - not bringing in a centre-back.

Southampton v Manchester United - Premier League

"Blind and Rojo, they're centre-backs who can play full back as well, but they're not out-and-out centre-backs. You look at it, and they look short."

United won the game against Southampton, but leaked two goals and required another top quality performance from David de Gea to ensure the three points went back to Old Trafford, suggesting there is merit to what Carragher was saying.

But after his relegation joke, many Twitter users were quick to point out that Liverpool have been struggling far more. Carra himself is happy to see Brendan Rodgers continue to be backed, though, at least for the time being.

"They (Liverpool) decided to give him the benefit of the doubt at the start of the summer. If you give him that much money, then after eight or nine games why make the change?," he asked on Sunday.

"In their last 18 games, they haven't scored more than one goal and that's the problem for a manager who is attack-minded like Brendan Rodgers."

Manchester United v Liverpool - Premier League