Stoke manager Tony Pulis believes the scale of Sir Alex Ferguson's achievements will only be widely recognised when the Scot leaves the club.
Ferguson has won 20 trophies, including two European Cups and 10 Premier League titles, during his 22-year stint at Manchester United.
United face a resurgent Stoke side on Saturday and Pulis said: "The manager is a living legend but I do not think people will give Alex real credit until he has left Manchester United."
He added: "You look at their success and the way he changed things around. Perhaps he was fortunate because he was given the opportunity at the time to clear everything out.
"It took him three or four years to do that and then he built a base. The club has gone from strength to strength.
"Along with Matt Busby, they are the two greatest managers in its history."
Stoke are riding high after coming up from the Coca-Cola Championship, gaining an unexpected victory against Arsenal in the process.
They are currently 12th in the table and clinched a place in the quarter-finals of the Carling Cup last Tuesday.
Pulis is relishing the challenge of facing United and has promised they will not change their approach at Old Trafford.
"This is one of games you pick when you get promoted. We know it will be a tough game but we have got to enjoy it. We are on a good run and confidence is high."
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