Eto'o played all 90 minutes of his country's FIFA World Cup opener against Mexico on Friday, which saw the African nation disappoint en route to a 1-0 defeat.
However, the Chelsea forward has since been unable to train.
Speaking at a press conference on Sunday, Cameroon coach Volker Finke revealed that his star striker had been troubled by a knee complaint since the end of the Premier League season.
"He has a problem with the right knee that he already had at the end of the season with his club Chelsea," said Finke.
"He did not train yesterday nor today."
Cameroon face Croatia on Wednesday in Manaus and will hope their record goalscorer is fit as they seek to keep alive their hopes of progressing from Group A.
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