MADRID, May 13 (Reuters) - Barcelona's Argentina defender Gabriel Milito could be out for up to nine months after surgery to correct a damaged knee ligament revealed a complete tear, the club said on Tuesday. The initial prognosis had been that Milito would be sidelined for six months, for an injury he picked up in the Champions League semi-final second leg defeat against Manchester United last month. 'First we carried out arthroscopic surgery on the right knee and discovered that effectively the ligament was broken, so we proceeded to extract tendon from the left knee to incorporate it into the damaged one,' surgeon Ramon Cugat told a news conference. 'Normally an operation on the anterior cruciate ligament would entail a six month layoff. In this case, we have replaced one ligament that was operated on seven years ago... and put in a new one. 'The six months could stretch to nine as this type of operation could have more complications.' The 27-year-old tore the same ligament in 2001 and that injury was controversially cited as the reason for a failed move to Real Madrid in 2003, though Barca have said the injuries are not connected. The Primera Liga's third-placed club signed Milito from Real Zaragoza for a fee of up to 20.5 million euros ($31.67 million) last July and he has been a key player in their defence this season.