Rabat claims that the 19-year-old chose to join Rafa Benitez on Merseyside for financial gain, and not to further his footballing ambitions.
Ngog endured a difficult season for PSG last term, he found the net only once in Ligue 1 and was left out of his side’s Coupe de France final panel.
Rabat claims that Ngog is lacking in self-confidence and is in the throes of some physical problems.
He told France Football: "David's choice is a more economic one than a sport one. He won't be given a lot of time to play. He will have to fight to impose himself. Him leaving PSG so young is a shame.
"He has been lacking self-confidence for a year now and he didn't score a lot. Pauleta's departure offered him a chance to play his part, but he had some bad experiences.
"For example he was not part of the squad for the Coupe de France final, I think this hurt him.
"He was ahead compared to the other players when he was 16, 17 years of age, but now… He grew up a lot in a short space of time and has had lots of physical and muscular problems that slowed him down.
"And don't forget that a certain Fernando Torres will be above him."
