Their quest for five titles is taking a huge toll on Ferguson's resources with 11 senior players nursing one ailment or another and matches almost every few days.
The game against Spurs -- who they also meet in the League Cup final on March 1 -- was one of the highlights when the draw was made three weeks ago, but is now more likely to resemble a reserve team fixture.
Spurs manager Harry Redknapp has already apologised to fans for planning to field a weakened team as he also wrestles with injury problems and a fight against relegation from the Premier League.
But Spurs' injury woes are almost nothing compared to United's, who lost three more players on Tuesday when they beat Derby County 4-2 in the second leg of their League Cup semi-final for a 4-3 aggregate win.
Midfielder Anderson and defender Jonny Evans were both ruled out for three weeks on Friday after both injured ankles against Derby.
Nani also suffered a groin strain and will miss the FA Cup tie, joining Rafael da Silva (hamstring), Wayne Rooney (hamstring), Patrice Evra (toe), Rio Ferdinand (back), Wes Brown (ankle) and Park Ji-sung on the sidelines.
Midfielder Owen Hargreaves has not played September because of a knee injury.
"We still have Dimitar Berbatov, Carlos Tevez and Cristiano Ronaldo fit, but we will have to call on some young players," Ferguson said on Friday.
FOUR MORE
"It's not the best position to be in. January is a difficult month. We've talked a lot about having a winter break to get rid of the strains you pick up in January. But it's not to be."
United have already secured the World Club title this season, and are still in the hunt for the Champions League, the Premier League, the FA Cup and the League Cup, but risk missing out on all of them because of the injuries to key players.
Defender John O'Shea, however, said United still had a clean sweep in their sights.
"We are aiming for all four," he said. "It is something you hope for at the start of the season. There is confidence these four trophies are up for grabs and when you have the players and belief we have, then why not ?"
After playing Spurs on Saturday, United visit West Bromwich Albion next Tuesday and are at home to Everton six days later.
They play their match-in-hand over Liverpool against Fulham the week after next, but will have to re-arrange the match against Portsmouth which clashes with the League Cup final.
That Wembley date comes between the two legs of their Champions League games against Inter Milan.
