Rio Ferdinand's injury problems and serious errors have led to some questioning his inclusion in the side. Should Ferdinand's problems continue during the World Cup, someone will have to step up to replace him. Who will partner John Terry? So far Fabio Capello has been unable to find an obvious replacement despite having a seemingly good talent pool. We take a look at the players who may be called upon to step in..

Joleon Lescott – Lescott became the second most expensive defender in English football in the summer when he moved from Everton to Manchester City. However he has struggled to live up to that £24m price tag. He has been a great threat from set-pieces throughout his career, something that could really help him hold down a starting place in the side. The main issue with his game seems to be his concentration.

Matthew Upson – Upson has been on the verge of the England side for most of this year and has sought to take his chances while Ferdinand has been injured. Although now a crucial part of the West Ham defence he has not been particularly special for England and has looked shaky at times. The fact that he is left-footed is an advantage since it would complement Terry. He does seem to lack pace and, although Capello does seem to like him, it could hold him back.

Gary Cahill – The Bolton defender left Aston Villa last January in search of first team football and it looks like the best decision he could have made. At Bolton he has become a crucial part of the defence and could very well earn a position in the national team based on this form. His determination to succeed after being unable to break into the Villa first team will endear him to Capello and his game will undoubtedly continue to improve. Whether his international experience will be an issue - with so few games before the World Cup - remains to be seen. He will need to make an impact if he gets any game time in the next two games.

Ledley King – Ledley King has been blighted by injury during his career. He currently is battling knee problems that are so bad he can only play matches and not train. His ability is unquestionable and he could have been even better had he avoided injury. If he can find some fitness and regular game time then he could be called up, however it would be a massive gamble to take him to the finals.

Michael Turner – The effect that Turner's departure has had on Hull's defence is clear for all to see as they are now leaking goals. He has performed well this season and has been the standout player in an otherwise shaky Sunderland defence. He is also a major threat from set-pieces and has already scored for his new club. The main problem with him, like Gary Cahill, is his lack of experience.

Jonathan Woodgate – Much like his Tottenham counterpart Ledley King, Woodgate has been blighted by injuries. A very promising career, which saw him transferred to Real Madrid, has suffered constant setbacks. However when fit he is a dominant and experienced defender who is used to playing in big games. But again like King, it would be a gamble to take him to the finals.

Phil Jagielka – The Everton defender has been laid-off for a long period of time and won't be back anytime soon. He was exceptional at the back for his club last season and if he can play to that standard when he returns then he could definitely receive a call-up. His last international was against Spain where he impressed, except for a mistake which gave Spain a goal. If he can get back to fitness and form in time he has a chance of being taken to South Africa.

Curtis Davies – Curtis Davies is currently sidelined with a shoulder injury and will not return for a couple of months. He has the unfortunate record of being the most expensive English defender not to represent his country. However he looked good before his injury and proved his aerial ability with his goal against Liverpool. His size and physicality will be a welcome addition to the England squad if he can return to form after his lay-off.

Ryan Shawcross – Another young and promising defender. He will undoubtedly go on to represent England, however whether this will be his year is less certain. Playing for Stoke will not be a disadvantage as Capello has already said he picks on form, however a lack of European may hinder him as he has no experience of high pressure continental games.

Jamie Carragher – Jamie Carragher infamously announced his club retirement several years ago. However he has been a consistent performer in Liverpool's defence. He will need to overcome his current dip in form, but should Capello's defensive worries grow he could call on the Scouser's never-say-die attitude and huge experience. The chances of him playing are slim.