The striker, who has been standing in for crocked striker FernandoTorres, was hit in the face by a ball during a training session on Sunday.
He has been ruled out his country's Under 21 European Championship qualifier against Slovenia on Tuesday night.
Nightmare: David Ngog is the latest Liverpool player to suffer injury
Big chance: Andriy Voronin
Torres, in the middle of three-week programme to overcome a groininjury, is almost certain to miss the visit of City onSaturday.
Steven Gerrard is suffering with a similar complain but hopes to be fit, while Benitez's attacking options have also been diminished by the absence of Albert Riera and Yossi Benayoun, who both have hamstring injuries.
Ngog has proved a useful bit-part player this season, scoring four in nine games for the Reds and winning the penalty which helped Liverpool to a 2-2 draw against Birmingham last time out.
Ukrainian misfit Andriy Voronin may be called upon to face City, or Benitez could turn to Dirk Kuyt or Ryan Babel, who he has tended to play in wide positions.
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