The 39-year-old has endured a difficult campaign so far, first breaking two fingers in a pre-season game with Bayern Munich that condemned him to two months on the sidelines.
Then the veteran Dutchman was ruled out for another five weeks following a collision with Everton's Tim Cahill at Old Trafford in November.
Superman: Van der Sar was sorely missed in the early part of the season
Van der Sar was nearing the end of his recovery when wife Annemarie suffered a stroke in her native Holland that meant her husband was granted a period of indefinite compassionate leave.
The former Ajax keeper finally returned to action against Burnley on January 16, since when United have not conceded a goal in the Premier League and reached the Carling Cup final by beating Manchester City last night.
Van der Sar's second-half save to deny Micah Richards when last night's contest was goalless was superb, and Ferguson was quick to highlight the contribution Van der Sar is making.
'Edwin makes a big difference,' said Ferguson. 'I know there are a few young keepers out there doing very well but Edwin is the best in the league just now.
'It is not just his saves, it is his composure and use of the ball. It is good to have him back.'
Man United 3 Man City 1 (United win 4-3 on agg): Rooney finds killer blowVIDEO SPECIAL: Who is better? The moments that made Peter Schmeichel and Edwin van der Sar Manchester United legendsVan's the man for me: Evans hails the return of United goalkeeper EdwinSunderland and Stoke in £4m fight for United goalkeeper Ben Foster Van der Sar's vigil for sick wife: United keeper by stroke victim AnnemarieMANCHESTER UNITED FC