The Chelsea midfielder Oriol Romeu has been ruled out for up to six months after undergoing surgery on the knee he injured during Saturday's Premier League victory at Sunderland.

The Spanish midfielder, who has made 33 first-team appearances since signing from Barcelona in the summer of 2011, underwent an operation on Monday to reconstruct the anterior cruciate ligament and repair the medial meniscus in the joint. The 21-year-old had suffered the rupture midway through the first half at the Stadium of Light and had returned to London immediately after the game, while his team-mates flew to Japan for the Club World Cup.

The surgery appears to have been successful, though the midfielder's rehabilitation will extend well into the latter stages of the current campaign and potentially leaves Chelsea short of numbers in central midfield. Frank Lampard, who returned from his own calf complaint at the weekend, did train with the squad in Yokohama and should play a full part in the two-game tournament.

The teenage midfielder George Saville, who had not been included in the original 23-man squad, has been formally included in Rafael Benítez's party and will wear the No56 shirt if required. Chelsea play their semi-final against the Mexican club Monterrey on Thursday, with their opponents having similarly drafted in Salvador Jasso for the injured César de la Peña.