The United centre-half has been plagued by fitness problems since injuring his back during United's Fifa Club World Cup campaign in Japan last December and has appeared in just 27 of 55 subsequent United fixtures.
Related ArticlesAncelotti: United fixture just another gameFerguson: no rift between Kuszczak and Van der SarKevin Garside: Manchester United search for rhythm as Chelsea peakWhen team-mates fall outDeco: Didier Drogba better than Wayne RooneyMan United fear Rio Ferdinand back problemFerguson said that United were now aiming to discover whether the root of Ferdinand's problem could be traced to his lower back. The club remain hopeful that the issue can be resolved without surgery.
But with United team-mate Wes Brown revealing earlier this week that Ferdinand's back has been a cause for concern for ''a very long time'', Ferguson conceded that his lack of progress was becoming a concern.
Ferguson said: 'We don't know exactly where Rio's calf injury is coming from whether it is from the lower back or a nerve thing.
'Calf injuries usually clear up within two-three weeks, but there is no sign of Rio recovering from the injury.''
With Nemanja Vidic due to return from his own calf problem, Ferdinand's absence at Chelsea is set to be covered by the Northern Ireland defender Jonny Evans.
Due to injuries and suspensions, Ferguson has been fielded a remarkable 15 different defensive permutations in 18 games this season. The United manager will make it 16 in 19 if, as expected, he selects of a back five of Edwin van der Sar, Brown, Vidic, Evans and Patrice Evra for the first time this campaign at Chelsea.
But he insists that the lack of continuity is no excuse for defensive failings that have seen United concede 15 goals in all competitions.
Ferguson said: 'It didn't affect us last year when we had the same situation. When we were on a run of 14 clean sheets, we were changing the back four all the time.
'We are lucky that we have a good squad, so I don't see why it should be the problem. But you also have to look at the number of goals scored in the Premier League this season as opposed to previous seasons.
'It's the most there has been, so maybe we are just in one of these unusual seasons. But it would be nice to get clean sheets, no question about that. It makes a difference.'
Ferguson said that Carlo Ancelotti's success so far at Chelsea was merely what he expected of the former AC Milan coach, who twice plotted United's Champions League downfall during his reign at San Siro.
'Ancelotti was a great coach at AC Milan and he has brought a wealth of experience with him to Chelsea.
'He's won the European Cup twice and was also a great player. I thought he'd do well. He has a very experienced team and, when you have that, it's maybe easier to adapt to new tactics or a new system.'
United have won just once in their last 10 visits to Stamford Bridge, but Ferguson insisted that Sunday would not prove decisive in the title race. 'It doesn't come into my thinking that it is a league decider, but it could be an important game.
'Towards the end of the season, you might think, 'I'm glad we got a result at Chelsea,' but it's difficult to pinpoint the importance of Sunday in relation to where we'll be in May.'