MANCHESTER: Sir Alex Ferguson has admitted Manchester United have been unable to diagnose the cause of a worrying injury that is continuing to sideline England defender Rio Ferdinand.

The centre-back, who turns 31 on Saturday, has been absent for two weeks with a persistent calf injury that has failed to respond to treatment and which, so far, has defied diagnosis.

"We don't know exactly where the calf injury is coming from," said Ferguson, who marked his 23rd anniversary as United manager here on Friday. "Whether it is coming from the lower back, whether it is a nerve thing.

"But calf injuries are normally two to three weeks and he is showing no sign of recovering from that injury.

"We're looking at the whole situation, whether it is coming from the back or not, but in the meantime he is out. There is no improvement so obviously we are looking at something different."

The form of Ferdinand has been under close scrutiny this season with some erratic performances for the Premier League champions and England raising question marks about his standing as an automatic choice for the national team.

Ferguson, who will partner Jonny Evans with Nemanja Vidic in the heart of his defence at title rivals Chelsea on Sunday after the latter passed a fitness test, has even publicly questioned whether Ferdinand will be selected for the England team in next year's World Cup Finals in South Africa.

Now, the prospect of Ferdinand being forced into an extended spell on the sidelines at Old Trafford must cast an even larger question mark over his international prospects and, possibly, United's own quest for honours.

In the meantime, Ferguson is clearly showing no signs of slowing down or losing his enthusiasm for the most high-profile job in club football, despite his age.

"I don't think so," said the 67-year-old Scot when asked whether his desire is lessening with the years. "Whether other people judge it differently, I don't know.

"But in the main, I have been lucky. I have had my health and that's all you can be thankful for.

"As you get older, there are different deteriorations in health, maybe small things, you can't avoid it. But, as I say, I have been lucky."

Ferguson's competitiveness will certainly be at the fore when United take on Carlo Ancelotti and his table-topping Chelsea at Stamford Bridge on Sunday, aiming to end a run of seven Premier League games without a win at the ground.

"Chelsea is a big game and, hopefully, we can improve our record down there because it has not been good the last six years or so," said Ferguson, whose side are currently two points behind the Londoners.

"I think the players realise it is a big game and their performance is going to be important.

"Carlo Ancelotti was a great coach at AC Milan, and a great player also, and brought a wealth of experience and knowledge with him.

"I thought he would do well but the change he has brought is a different system to what Chelsea used in the past and it seems to have worked well.

"They have a very, very experienced team and when you've got experienced players maybe it is easier to adapt because, generally, they have more tactical knowledge than younger players. That's maybe been a bonus to them.

"The two years Chelsea won the league (2005 and 2006), they got off to great starts and caught us all on the hop.

"We had to change our ideas about pre-season training because they had such good starts."