England captain Rio Ferdinand could be ruled out of the World Cup after suffering a knee injury in training. The 31-year-old centre-back had a scan after hurting his left knee in a challenge with striker Emile Heskey during Friday's first training session. Ferdinand is seeking a second opinion, but a Football Association source has told the BBC "it's not looking good". The FA is due to confirm the seriousness of Ferdinand's injury later in a statement. Ferdinand emerged from hospital on crutches and Tottenham's Michael Dawson, who has been put on standby, could fly to South Africa in the next 12 hours if the Manchester United defender is definitely ruled out. If Ferdinand is out, Liverpool midfielder Steven Gerrard will step into the captain's role to lead the side at the World Cup. The news could be a devastating end to an already disappointing season for Ferdinand, who, because of a back injury, had been restricted to only 13 Premier League starts for Manchester United and 21 appearances overall. He had appeared to have overcome his injury problems as he trained with no setbacks at England's two-week World Cup training camp in Austria and came through the recent friendly victories over Mexico and Japan. But Ferdinand sustained yet another injury at the end of England's first training session since arriving in South Africa, leaving manager Fabio Capello to break the news in a press conference. "The bad news is in the last moment of the training, Rio suffered a problem with the knee," said Capello. "It was a normal tackle. He turned and then he had a problem. He has gone to hospital and we will have to wait for the scan." Should a player be ruled out of the World Cup more than 24 hours before the kick-off of their first match, a replacement can be drafted into the 23-man squad. This happened in 2002 when England midfielder Danny Murphy broke a bone in his foot and Trevor Sinclair was called up. However, if a player is injured during the course of the tournament, a replacement cannot be brought in. England begin their World Cup campaign with their game against the United States on 12 June. Dawson was the only centre-back dropped by Capello when he trimmed his 30-man training squad to the 23 for the tournament. Friday had started on a more positive note for England, with the news that midfielder Gareth Barry - who had been a major doubt after suffering an ankle injury towards the end of Manchester City's season - trained with the squad. "Gareth Barry is improving every day," said the England boss. "In the afternoon he is training in the gym with the physio and in the morning always with the team. "He is not completely training with the team as we have to be very careful." Meanwhile, goalkeeper David James sat out the training session after suffering a slight knee problem. His injury is not thought to be too severe and is related to the 11-hour overnight flight from London to South Africa on Thursday. Source: BBC