Volker Finke's men are one of just three nations to have drawn a blank in their opening game to date, having gone down 1-0 to Mexico on Friday.
And leading scorer Eto'o now appears likely to miss Wednesday's crucial match against fellow winless side Croatia, dealing a further blow to Cameroon's hopes of progressing from Group A.
The Chelsea frontman had been bothered by a knee injury in the closing weeks of the Premier League season, and it is that complaint that is set to rule him out.
"The doctors confirm! I would probably not be able to play the match vs Croatia on Wed, june18th due to an injury on my knee causing me pain," Eto'o revealed on his official Twitter account.
Cameroon coach Finke had already confirmed Eto'o failed to train over the weekend, and his absence means one of Vincent Aboubakar or Fabrice Olinga are likely to be called upon in Manaus.
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