Christian Burgess is expected to be out of action for four weeks.
A scan has revealed ligament damage to the right ankle he injured against Hartlepool last weekend.
As a result, at present it is anticipated the central defender will miss the next month of Pompey action.
It represents a blow to the Blues, who now have only two match-fit centre-halves on their books in Adam Webster and Matt Clarke.
The young duo will start Saturday’s trip to table-topping Northampton and are pencilled in to partner each other going into the new year.
Jack Whatmough is not, however, currently a consideration after returning from a cruciate knee ligament injury.
The Gosport youngster has still to play any sort of competitive football and recently was pulled out of a scheduled appearance for the reserves against Bournemouth.
Regardless, Whatmough is making pleasing progress and taking part in full training as he builds up his match fitness once again.
In the meantime, Clarke is available for Pompey’s next four matches until his loan spell from Ipswich is up for renewal on January 3.
That is only six days before Pompey travel to Portman Road in the FA Cup, with Clarke unable to play against Paul Cook’s side because he is cup-tied.
In the meantime, Burgess has started coming back from that tackle by Mikael Mandron.
Before the injury, the Peterborough recruit had been an ever-present in the League Two campaign under Paul Cook.
He has totalled 23 appearances so far.