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The Portugal star suffered the problem during Wednesday's Champions League encounter with Braga and means he will miss Saturday's trip to Aston Villa.
"Nani is out with a tight hamstring. It will be about 10 days," said Sir Alex Ferguson.
Central defender Jonny Evans is struggling with a groin injury for this weekend's match, which could mean a second game in four days for Chris Smalling.
"Jonny is a doubt," said Ferguson. "It is not a serious one. We are monitoring it and hopefully he will be OK."
If recent history is anything to go by, United should not be too inconvenienced given they have recorded 10 wins and six draws in the 16 years since their last Premier League defeat at Villa Park, in 1995.
It seems strange to think now the Midlands giants were the side United edged out to win their first title of the Ferguson era in 1993.
The Red Devils have won 11 more, a figure almost matched by Villa's turnover of managers, which, following the summer arrival of Paul Lambert, stands at eight since the exit of Ron Atkinson in 1994.
"It is a daunting task to bring Aston Villa back to where everyone expects them to be," said Ferguson. "They have a great history. They won the European Cup in 1981. It is correct to say they have not matched their expectations over the years.
"You wonder whether changing managers has a bigger impact than the actual playing standards of the team. They have had quite a few managers and it takes time to change a team's fortunes."
Source: PA
Source: PA