The England midfielder made his first start in more than two years against Wolves last month but tore his left hamstring after only five minutes.
United have now sent him to Munich for treatment but Ferguson knows time is running out for Hargreaves, who will be 30 when his contract expires in the summer.
Frustration: Owen Hargreaves leaves the pitch injured against Wolves last month
'We are all concerned,' said the United boss.
'He hasn't played for over two years and we would have to be super-optimistic to think everything is going to be rosy for him in terms of him making a comeback.
'What we're trying to do is help the lad. That in itself is our biggest challenge, to help him get back to a level where he can play football.
'This hamstring injury is a real kick in the teeth to him because, upuntil then, he had trained so well ahead of the Wolves game.
Concerned: Ferguson speaks to reporters on Monday
'This was a complete tear and it can be six to eight weeks.'
Hargreaves has made just 26 starts since moving to United from BayernMunich for £18million in 2007 because of chronic tendinitis in both knees.
Having invested nearly £35m in a player who earns £80,000 a week, United were believed to be considering a pay-as-you-play deal but even that looks unlikely now.
Asked about the contract situation, Ferguson said: 'That's not something we're thinking about at this moment in time.'
Ferguson will send out a full-strength side against Valencia at Old Trafford on Tuesday night, even though both teams have already qualifiedfor the knockout stages of the Champions League. United need to avoid defeat to finish top of Group C and avoid the possibility of drawing Barcelona, Real Madrid or Bayern Munich in the last 16.
They can also become the first club ever to qualify without conceding a single goal in their six group games.
'We've played younger players in the group stages and they haven't made any mistakes,' said Ferguson, referring to the contrast of their 4-0 Carling Cup defeat at West Ham last week.
'I think in European games they realise the concentration has to be 100 per cent.'
After Saturday's game at Blackpool was postponed, Valencia provide a chance for United to face a strong team ahead of their upcoming Arsenal and Chelsea games.
Happier days: Hargreaves in action for United against Chelsea in the 2008 European Cup final in Moscow
Ferguson said: 'I'll play my strongest side and then we have six days to prepare for the Arsenal game.
'But we're going to concentrate on Valencia first. I think we are going to see a team who will come here and be very difficult to beat.'
Michael Owen is ruled out despite returning to training last week after another period on the sidelines with a hamstring injury.
'Michael has no chance of being involved,' said Ferguson.
'He returned to training in the middle of last week but we are going to make sure he gets two more weeks behind him. We don't want to take any risks because it was probably our fault he was injured last time.'
Enlarge Meanwhile, Ferguson was derisive over calls for reform of FIFA in thewake of the World Cup voting scandal. He admitted he was as shocked as anybody to see England's bid win just two votes.
But asked whether he agreed with demands for an overhaul of world football's governing body to make it more democratic, Ferguson said: 'That's a new one for FIFA - democracy! I said at the time the disappointing thing for me was only getting two votes. That's why everyone was so shocked.
'I see all these things about changing FIFA, but how can you change it? Is that 22-man committee going to change if you ask them? Give me a break.'
Hamstring injury knocks out United midfielder Hargreaves for five weeksOwen boost for Man United as striker closes in on Christmas comebackMANCHESTER UNITED FC