Nani, sidelined by injury since the Champions League tie with Braga on November 7, has yet to sign a new deal after talks stalled in the close season.
"Obviously his people have been negotiating with [chief executive] David about a new contract but I don't know what stage we are at with that. But we won't be letting him go," Ferguson told the club's website.
"He's injured. His contract's not up for a year-and-a-half."
Ferguson said he still regarded 26-year-old Nani, who joined United from Portugal's Sporting in 2007, as an important part of his squad.
"We need a Nani. He offers something different from the other players. He's an incredible talent, the boy's a great talent. Unfortunately, he's injured at the moment so we sent him over to Dubai for a break and hoping a bit of warmth will help him in his recovery.
"He's not far away but he's far enough away. I don't think I'll get him back until probably the middle of January. It's a hamstring injury he's got.
"A player like Nani has never had a hamstring injury ever - I don't think he's ever had many injuries with us. But he's got it right in the top and it's a bad one."
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