The Portugal winger was hurt in Wednesday's 3-1 win over Braga in the Champions League but Ferguson hopes he may only have to miss the weekend trip to Aston Villa.
"Nani is out with a tight hamstring. It will be about 10 days," said Ferguson at his pre-match press conference, allaying fears the injury could have been a tear.
Ferguson also revealed doubts over defender Jonny Evans, who sustained a groin injury in the same game and had to be substituted.
"Jonny is a doubt," added Ferguson. "It is not a serious one. We are monitoring it and hopefully he will be OK."
If Evans misses out, Chris Smalling could continue his comeback from injury with a second game inside four days despite the United boss previously saying he might be rested.