England captain has had an injury while training with the rest of the English squad at their training camp in South Africa.
Ferdinand has been taken off to hospital to have a scan on his left knee.
The defender suffered his injury during the last few minutes of this morning's training.
The England manager Fabio Capello and the rest of the team are unsure of the severity of the injury.
Capello has said: "Rio picked up a knee problem in training. It happened near the end.
"He has gone to hospital for a scan and we will have to see how things are."
Ferdinand has been dogged with problems at Manchester United all season, and was looking foward to an injury free world cup.
This injury may not be to bad, which could mean he can start training again and play in the group matches.
However, if the injury is worse than hoped, he may miss his chance to take England to victory.
Number one goalkeeper David James also missed training today, but Capello is not concerned with him and has said, he "only a slight knee problem".
However, the suprise pick has had an injury lately and it was unsure if he would be fit to play. Capello has taken him to South Afric and was training on the pitch today.
Everyone in the England camp is hoping Rio will be fit to take part in England's Group C opener against the United States in Rustenburg on June 12.