Jonny Evans is set to be out for two weeks to add to Manchester United's defensive problems.
The Northern Ireland international was United's only fit central defender against Wigan on Boxing Day but was replaced at half-time.
Afterwards Ferguson - whose side ran out 5-0 winners over the ten-man Latics to move level on points with Man City - said Evans faces a spell on the sidelines.
The United boss said: "Jonny came off injured. He has a calf injury and will be out for two weeks.
"We have had a nightmare the last few days. Phil Jones and Chris Smalling have both been ill. Rio Ferdinand called off yesterday with a back injury and Jonny Evans has come off at half-time.
"It does put you under pressure."