Ferguson endured a torrid spell with goalkeepers in the six-year period between Schmeichel's departure from Old Trafford and Van der Sar's arrival from Fulham in 2005.
Outgoing: Van der Sar is set to announce he will retire at the end of the season
Golden gloves: Schmeichel makes one of his finest saves at Tottenham in 1994
From the calamitous Massimo Taibi tothe erratic Fabien Barthez, United went through no fewer than 10keepers before eventually finding a safe pair of hands in the Dutchman.
Ferguson said: 'I should have signedVan der Sar after Schmeichel. I made a mistake but now we think we knowwhere we're going. Schmeichel and Van der Sar have been magnificent forus. I've been very lucky to have had them both.'
Newsigning Anders Lindegaard has been training with United for severalweeks but the Denmark keeper will not be registered to play until afterthe game against Stoke on January 4.
However, Lindegaard is expected tostart out as a back-up goalkeeper to compete with - or replace - TomaszKuszczak, with Ferguson set to sign a permanent successor to Van derSar in the summer.
Unitedappear to have narrowed the list of candidates down to Manuel Neuer ofSchalke, Atletico Madrid's David de Gea and Ajax keeper MaartenStekelenburg.
There has been speculation about Van der Sar's future since he was 40 at the end of October, although at the time the Dutchman dismissed as 'rubbish' comments from United goalkeeping coach Eric Steele that this season would be his last.
'I expected it to be his last season,' said Ferguson. 'It's amazing to be playing at 40 years of age at our level. You can only do it if you've got that structure and the mind. To be able to programme himself and train the way he does is phenomenal. He's an outstanding man.'
United kick off a run of four games in 10 days at home to Sunderland on Boxing Day but Ferguson is just as concerned about finding a date to re-arrange last weekend's postponed fixture at Chelsea.
He admitted the decision suited Carlo Ancelotti's side, who are without a win in five Premier League games.
Ferguson said: 'Obviously Chelsea are in a bad bit of form and they were probably quite happy to get the game off. 'We've had to look at the fixtures in the last few days and it's quite a difficult one. I'm not concerned when it's played as long as it's fair on both teams in the sense of fixture congestion. It certainly seems like it's not going to be played until after March at least.
'My aim is to be top by January 4. By that time the top of the league will have taken shape but, at the end of the day I don't think they'll be anyone outside of ourselves, Chelsea or Arsenal. Manchester City have a chance. They've also got the Europa League but maybe they can buy in January.'
Ferguson is wary of a Sunderland side who stunned Chelsea 3-0 at Stamford Bridge last month and came within seconds of beating United last season.
However, former Old Trafford skipper Steve Bruce has not won against his old club in 15 attempts as a manager, and will be without on-loan United striker Danny Welbeck, who is ineligible. Meanwhile, it has been confirmed that United will play Barcelona in a pre-season friendly in Washington DC on July 30.
While Manchester United have at times struggled to find their touch in front of goal this season, their former star Cristiano Ronaldo has suffered no such problem.
A hat-trick in Real Madrid's 8-0 destruction of Levante in the Spanish Cup on Wednesday night took his tally to a stunning 25 goals in 25 games for the season. It puts the Portugal forward on course to beat his previous highest goalscoring total of 42, achieved in the 2007-08 campaign at United.
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