Sir Alex Ferguson produced a surprising answer when he named the only player that would be guaranteed to feature in his all-time greatest team during his time at Man United.

In nearly three decades at the helm, the Scotsman presided over multiple distinctly successful eras at the club, winning the Premier League 13 times and lifting the Champions League twice.

During that time some of the most legendary names in the history of English football plied their trade at Old Trafford, including the likes of Eric Cantona, Cristiano Ronaldo, Wayne Rooney, Ryan Giggs and Paul Scholes.

After ending his managerial career on a high in 2013 by leading United to what remains their most recent league title, Ferguson was asked by to name the greatest XI of his time at the club.

And while he admitted a number of legendary names immediately sprang to mind, it was a more surprising player that he insisted was the only certainty to have a place in the team.

Sir Alex Ferguson revealed only one Man United player would be a guarantee for his all-time XI

'It is absolutely impossible.' He told the Sunday World.

'You look at the strikers I had, going back to [Brian] McClair, [Mark] Hughes, [Andy] Cole, Cantona, [Ruud] van Nistelrooy, [Louis] Saha, Ole Gunnar Solskjaer, [Dwight] Yorke, Teddy Sheringham.

'Then to the present players -- Wayne Rooney, [Robin] van Persie, Chicharito [Javier Hernandez]. How do you pick out of that? Cantona and somebody else maybe?'

'The midfield was probably a bit easier because you have [Roy] Keane, [Bryan] Robson and Scholes -- they were fantastic players -- and [David] Beckham, Ronaldo and [Ryan] Giggs.'

However, in his autobiography Ferguson insisted there was a guaranteed place in the full-back position for Denis Irwin.

'Honestly, I would say Denis Irwin would be the one certainty to get in the team. I called him an eight out of 10.

'At Highbury in one game, he had a bad pass back in the last minute and [Dennis] Bergkamp came in and scored.

'After the game, the press said: "You must be disappointed in that pass back." I said: "Well, one mistake in 10 years isn't bad." He was an unbelievable player.'

After joining United from Oldham in June 1990, Irwin would go on to become one of the most decorated players in Irish football history, winning 18 trophies across 12 seasons at the club.