Fabio Capello has revealed that Rio Ferdinand has suffered a knee injury in training and the defender is now having a scan at a local hospital.
The England players were put through their paces for the first time at their Rustenburg training base in South Africa on Friday, with the World Cup now fast approaching.
The Three Lions will get their quest for global glory up and running against United States on 12th June and are desperate to head into the tournament with their strongest side available.
That includes national skipper Ferdinand, with the Manchester United centre-half set to marshal the heart of their back four.
However, his involvement is now in doubt after he picked up an untimely injury in training and Capello's concern is such he has placed Tottenham's Michael Dawson on stand-by.
Capello claims the problem is not too serious but admits he faces an anxious wait on the results of a scan before the picture becomes any clearer.
"The bad news is that in the last moments of the training session Rio suffered a problem with his knee," said the England boss.
"He has gone to hospital for a scan. We will have to see how things are. He did it during a tackle.
"It was a normal tackle, but there was a slight problem. It was his left knee. We have to wait for the scan, but we have Michael Dawson on call."
Capello also confirmed that goalkeeper David James was forced to sit out the session after picking up a slight knock.
"David James has a little problem with his knee, but it is not a big problem," he said.
On a more positive note, Gareth Barry was able to take part in training for the first time since injuring his ankle towards the end of the domestic season.
England will not be rushing the Manchester City man back ahead of schedule, but Capello admits he is pleased with the midfielder's progress.
"Barry has started to train with the team and he has improved every day," said the Italian.
"He is getting better and better every moment and in the afternoon he will train in the gym with the physio. Every morning he will be with the team. We have to be careful because the ankle is still an issue."