John Obi Mikel has pulled out of Nigeria's World Cup squad as "he did not want to put his career at risk" by playing with a knee injury.[LNB] The Chelsea midfielder has been struggling to shake off the problem after undergoing surgery in recent weeks.[LNB]He was rested for a match against Colombia and trained with his international team-mates on Friday - however, he confirmed on Saturday he has withdrawn from the tournament in South Africa, which starts next Friday.[LNB]Mikel explained: "It is the same knee and I did not want to risk any further damage, also I think it is in the best interest of the team for me to withdraw rather than waste a slot."[LNB]The player had been closely monitored since joining his team-mates at their pre-tournament training camp on the outskirts of London a week ago, with coach Lars Lagerback repeatedly giving reassurances over his fitness.[LNB]On Friday, Mikel trained fully with the rest of the squad for the first time since they arrived at their World Cup base in Durban.[LNB]Saturday's news is a massive blow for the Super Eagles, although Mikel himself is backing them to excel when they begin their quest to qualify from Group B against Argentina on Saturday.[LNB]He added on Kick Off: "I wish the team good luck and I know that they will surprise people."[LNB]Lagerback has acted immediately to replace Mikel, adding uncapped Sochaux midfielder Brown Ideye to his squad.[LNB]The 21-year-old was one of the unlucky seven players to be dropped from the squad earlier this week when the Swede trimmed his list to 23.[LNB]Ideye is yet to play for the Super Eagles at senior international level, but has impressed during his time with the Under-20s, helping them to reach the quarter-final of the 2007 World Cup in Canada.[LNB]The Nigerians wrap up their preparations for the finals with a friendly against North Korea in Johannesburg on Sunday.[LNB]After their opening Group B match in Johannesburg, Lagerback's team face Greece in Bloemfontein on June 17 before wrapping up their group matches with a match against South Korea in Durban five days later.