Irwin played for Ferguson between 1990 and 2002, picking up seven league titles, a Champions League and numerous other trophies over that period, but was never considered one of the team's stars.
However, talking at an event in Glasgow to promote his recently released autobiography, Ferguson said:
People ask you who is your best Man United team. It is absolutely impossible. You look at the strikers I had, going back to (Brian) McClair, [Mark] Hughes, [Andy] Cole, [Eric] Cantona, [Ruud] van Nistelrooy, [Louis] Saha, Ole Gunnar Solksjaer, [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?
The midfield was probably a bit easier because you have (Roy) Keane, (Bryan) Robson and [Paul] Scholes, they were fantastic players and (David) Beckham, [Cristiano] Ronaldo and [Ryan] Giggs. But 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.
Considering for a large proportion of time at United so far that Frenchman Patrice Evra was considered among the best full-backs in world football, the comments say much for Ferguson's opinion of Irwin.
The man signed from Oldham for £600,000 may not have been quite the force of Evra in attack, but was tremendously reliable in his defensive work throughout his time at the club and was well respected within the dressing room.
Per ESPN, Ferguson recently described Ronaldo as the "most talented" player he had managed. Given that Ronaldo was the leading figure in the club's 2008 Champions League success that the Portuguese should not even be considered an automatic pick says much of Irwin's importance.
The same could also be said of being picked ahead of Cantona, whose 1992 arrival was the spark that helped United to extraordinary levels of success for the remainder of the decade.
The Guardian's Paul Campbell rated Irwin as United's fourth-best signing of the Ferguson era earlier this year—behind Peter Schmeichel, Cantona and Ronaldo, while numerous outlets put Irwin in their best team of that time period.
Ultimately, though, there can be no better judge than the man who oversaw United for all that time and, if he deems Irwin to be the first name that would have been on his teamsheet, there can be little argument.