Barrett picked up the injury in last week's reserve team clash against Bristol City.
Fears were initially expressed the youngster had broken his leg but an X-ray revealed the injury was not as bad as first thought.
But tests have still shown Barrett requires a break from football and it is likely to be early November before he can relaunch his claims for a first team spot.
Meanwhile, Australian defender Hayden Foxe is still three weeks away from a return to the Pompey team.
Foxe has not featured for the First Division pace-setters since the end of August and is recovering from a knee injury.
In other news, Portsmouth have released French midfielder Ulliano Courville from his contract by mutual consent, just over a year after joining the Fratton Park club on a free transfer.
Former Pompey boss Graham Rix originally signed the former Monaco player to a three-year deal in September of last year following his release from The Principality club in the summer of 2001.
Courville has yet to make a first team appearance this season under current boss Harry Redknapp and has been spending the majority of his time in the South Coast club's reserves.
