There have been suggestions that the Portuguese winger could be on his way out of Old Trafford after failing to command a regular starting place this season.
Arsenal have been linked with the 26-year-old, who is currently sidelined with a hamstring problem, but Ferguson has dashed their hopes.
"We won't be letting him go. We need a Nani," said the Scot.
"His contract isn't up for a year and a half. He offers something different from the other players. He's an incredible talent, the boy.
"Unfortunately, he's injured at the moment. We have sent him over to Dubai for a break, to help him in his recovery.
"He's not far away but he's far enough away. I don't think I will get him until the middle of January.
"He's got a future here. Why would I want to let him go?"