Vidic faces up to five weeks out
There was better news for fellow centre back Rio Ferdinand, who had been ruled out for six weeks with a hamstring injury but who could now return in time for the Premier League champions' home match against Arsenal on August 28.
Ferdinand and Vidic picked up the injuries in last Sunday's season-opening 2-1 victory at West Bromwich Albion.
"Vidic has a calf muscle injury which means he will be four to five weeks. It is difficult to say with calves but it is longer than we thought," Ferguson told reporters, referring to original expectations the Serb would be out for two weeks.
"Rio is not as bad as we thought so he could be ready for our game against Arsenal. I don't think he'll be ready for Monday but it is amazing how it can change in a few days."
"Rio has had issues with his back and hamstrings over the last two to three years and from time to time it surfaces," he added. "This is another occasion. Fortunately it is not a serious one so he should be back soon."
FAITH IN YOUTH
For United's home game against Tottenham Hotspur on Monday, Ferguson said that Jonny Evans, 23, and new signing Phil Jones, 19, would deputise in central defence, with Chris Smalling, 21, playing at right-back.
Although Patrice Evra is available to return at left-back after missing United's opening game against West Brom, Javier Hernandez and Darren Fletcher remain sidelined despite making progress in their recoveries from concussion and a virus.
Ferguson said he had faith in his "very young defensive lineup" against Spurs.
"Evans has ample experience - he has been at the club a long time since 12 years of age and has played plenty of first-team games," he said.
"Jones has got the experience of playing for Blackburn last season in the Premier League. He and Smalling I thought were the stars for the [England] Under-21s this year. Smalling had a fantastic season last year and has continued his progress this year."
It does not help that United also have an inexperienced goalkeeper in 20-year-old Spaniard David de Gea, who has made an underwhelming start at Old Trafford - conceding soft goals against both Manchester City in the Community Shield and West Brom last weekend.
"De Gea will learn from the experience of the games he has had so far," said Ferguson.
The Scot is renowned for developing young talent and said he was excited with this latest crop.
"It's a very young team at the moment but it is a team full of energy and with great ability and I am enjoying watching them."