Aston Villa boss Martin O'Neill believes Sir Alex Ferguson's record over more than two decades at Manchester United will "stand the test of time".
O'Neill will be hoping Villa can win at Old Trafford for the first time in 26 years in Friday evening's Barclays Premier League clash.
But, whatever the outcome, the former Celtic manager knows Ferguson's achievements with the Red Devils are unlikely to be surpassed.
O'Neill said: "Sir Alex is at the top of the tree. He has been a brilliant manager for a long, long time and proved himself every year for 20-odd years.
"Anyone who has been at a club of that standing for that length of time, and been as successful as he is, is fantastic. It just couldn't be repeated.
"Carlo Ancelotti mentioned he was at (AC) Milan for eight years and, in Italian terms, that really is a lifetime.
"For someone to be at a club the size of Manchester United year in year out, and be as successful, is phenomenal.
"He has built not just teams, but successful teams. Eventually you have to change teams if you are there for that amount of time. There is a certain lifespan and then you go onto develop another side.
"What has been so fantastic is Sir Alex has developed these teams and they have all been successful.
"He is now contesting Champions League finals on a more regular basis and winning championships. The record is there for everyone to see and will stand the test of time."
O'Neill insists Villa do not have "a mental block" about playing away to United despite their dismal record at Old Trafford.
Villa have already defeated Liverpool at Anfield and Chelsea at home this season.
But O'Neill does not believe his players have to overcome any psychological barriers if they are to get the better of United.
O'Neill said: "We keep being told we have not won at Old Trafford for 26 years but the whole point is we can't be affected by something that has happened when we have not been involved the majority of the time.
"In fact, in the last couple of years we have played well enough at Old Trafford to have got something out of the games.
"I am thinking particularly of an FA Cup tie a couple of years ago when we conceded a very late goal and last season in the league when we played exceptionally well and still lost right at the death.
"I don't think we have a mental block about going there despite our record at Old Trafford. We definitely have the ingredients and players to do well in the game and we will be going into it with plenty of confidence.
"We are playing very well at the moment and it is an opportunity for us to try and push on."
Central defender James Collins is included in the squad after recovering from injury but his replacement Carlos Cuellar has been in excellent form in recent games.