BILLERBECK, Germany (AP) _ Central defender Nemanja Vidic is out of the World Cup because of a knee injury, a severe blow to the beleaguered Serbia-Montenegro team.

Vidic, of Manchester United, injured his left knee when he slid on grass during a Monday training session as he was reaching for a ball and left the pitch in pain, team spokesman Aleksandar Boskovic said Tuesday.

Vidic, considered the team's biggest star, sprained medial collateral ligaments in his left knee, "putting him out of action for several weeks, which means he will miss the World Cup even if we reach the finals," Boskovic said. He said Vidic was in considerable pain.

Vidic had been expected to play against Argentina on Friday after missing Serbia-Montenegro's 1-0 loss to the Netherlands on Sunday. He sat out that game because of a one-match ban he received after a red card in qualifying.

Serbia-Montenegro plays in Group C _ considered the toughest group in the tournament _ which also includes Ivory Coast.

Vidic is a key player in Serbia-Montenegro's defense, which conceded only one goal in 10 qualifying matches. His return to the team was eagerly awaited for the match against Argentina as Serbia-Montenegro needs at least a draw to stay in contention.

"Vidic's injury is a big handicap for the team," said his teammate, midfielder Ognjen Koroman. "He's the best player of our generation. It will be very hard without him."

Making things worse, Serbia-Montenegro is the only team with 22 players at the World Cup, one less than the maximum allowed.

Defender Dusan Petkovic, the son of Serbia-Montenegro coach Ilija Petkovic, left the squad on the eve of the World Cup, saying he did not want his father to be "crucified" by the media in case Serbia-Montenegro dropped out of the competition after the group stage.

With Vidic's injury, Serbia would have only 21 players available, Boskovic said. "It's a big blow for our team."

Serbia-Montenegro has no adequate replacements for Vidic in the defense. Midfielder Igor Duljaj took his place against the Netherlands in defending against Arjen Robben, who scored the only goal of the match.

Formerly known as Yugoslavia, the team last qualified for the World Cup in 1998 and reached the round of 16 before being knocked out by the Netherlands. Serbia-Montenegro's appearance in Germany is the team's 10th, but the first _ and last _ under the new name, as Montenegro has voted for independence.