The Portugal international will miss games against Arsenal, Marseille and Bolton, adding to Ferguson's already lengthy injury list.
The Manchester United winger was forced out of his side's 3-1 defeat at Liverpool on Sunday, leaving the field on a stretcher after Jamie Carragher's first-half tackle left a deep gash on his left shin.
Many observers felt the challenge warranted a straight red card, rather than the yellow Carragher received, and the defender seemed to acknowledge he had had a lucky escape by visiting the United dressing room to apologise to Nani after the game.
Ferguson revealed the extent of the injury this morning, in his first press conference since being charged by the FA for comments made about referee Martin Atkinson in the wake of United's defeat away to Chelsea last week.
The manager is reasonably happy with Nani's progress, but still ruled him out of tomorrow's FA Cup quarter-final with Arsenal, Tuesday's Champions League clash against Marseille and next Saturday's Premier League meeting with Bolton.
"We have managed to stitch the injury, which is good news," said Ferguson. "We need to guard against infection and swelling.
"It is difficult to say exactly when he will be ready to return but we won't get him back for the Bolton match next Saturday."
Ferguson also confirmed that neither Rio Ferdinand nor Park Ji-sung, who are recovering from calf and hamstring injuries respectively, are set for a comeback this weekend.
Antonio Valencia's return is also on hold, although the Ecuador midfielder did feature in a closed-doors game earlier this week and Ferguson does not feel it will be long before he makes his comeback.
"Antonio is difficult to assess because he has been out for such a long time," said Ferguson.
"He has been training very well and is taking part in a few practice matches between the players.
"It is deciding when we can put him into the cut and thrust of a game. There is a question mark over the timing but at some point, we are going to have to try it.
"It won't be tomorrow. Hopefully it will be quite soon."
It means Ferguson could have up to 10 senior players missing tomorrow, although skipper Nemanja Vidic is back from suspension and the United manager has vowed to play his strongest team.
"A quarter-final tie against Arsenal is always an exciting game," he said.
"It is an opportunity to get to the semi-final, so we will be doing our best and we will play our best available team."
Liverpool captain Steven Gerrard out for a month after groin surgery
• Midfielder underwent surgery after consulting groin specialist
• Gerrard will miss crucial games for Liverpool and England
Steven Gerrard will miss Liverpool's attempt to salvage their Europa League campaign and England's European Championship qualifier against Wales this month, after undergoing surgery on a groin injury.
The Liverpool captain is expected to be out for up to four weeks after an operation to repair a problem that has plagued him since the win at Chelsea on 6 February. Gerrard featured in Liverpool's recent league games against Manchester United and West Ham but was still in discomfort after the 3-1 defeat of Sir Alex Ferguson's side last Sunday.
The midfielder missed Liverpool's 1-0 defeat in Braga on Thursday as he awaited a specialist's verdict on the injury and, as a result of the surgery, will be absent when Kenny Dalglish's team attempt to overturn the first-leg deficit at Anfield next week.
The 30-year-old will also miss league games against Sunderland and West Bromwich Albion, England's qualifier in Cardiff on 26 March and the international friendly against Ghana three days later. Gerrard is targeting Manchester City's visit to Anfield on 11 April for his comeback.
A Liverpool statement read: "LFC can today confirm that Steven Gerrard has undergone successful surgery on his groin and will be out for up to four weeks. The decision was made following consultation with a specialist."