Paraguay striker Oscar Cardozo, who scored 36 for Benfica this season, is in danger of missing Monday's match against world champions Italy in Cape Town.
The 27-year-old has failed to completely recover from an ankle problem sustained against Sporting Lisbon in April and has been training separately from the team.
Paraguay technical director Horacio Cartes said: "Unfortunately, Cardozo's injury is more serious than what we thought and we are worried."
However, Manchester City striker Roque Santa Cruz is training fully again having shaken off a knock to his knee.
ITALY midfielder Gianluca Zambrotta has welcomed the criticism that is being directed at the reigning world champions.
Marcello Lippi's team, who only arrived in South Africa on Wednesday, have been branded an "old side" by many, but the 33-year-old says the Italians will be formidable at the World Cup.
"We are used to criticism," the AC Milan man said. "It doesn't affect us. On the contrary, it unites us and creates a team spirit. We are not worried about it, it only motivates us. Back in 2006, few thought we would progress past the quarter-final stages and yet we proved them wrong."
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