On-loan duo David Wheeler and Andre Green are expected to be out for the remainder of the month.
The duo are currently sidelined through injury and are not anticipated to return to Pompey first-team action until November.
Green, on loan from Aston Villa, underwent a minor knee operation last month.
The procedure to remove a cyst from his right leg means he has not featured for the Blues since the 1-1 draw with Shrewsbury on September 8.
The winger has only made two Pompey appearances since arriving in the summer.
Fellow loanee Wheeler picked up an ankle injury in last weekend’s defeat at the hands of Gillingham.
It was originally estimated that the QPR man would be out for ‘at least a week'.
However, it has now emerged that the former Exeter man will be sidelined for at least another three weeks.
Wheeler came on as a second-half sub in the defeat to Gillingham.
Since his arrival from QPR, he’s made seven appearances, scoring one goal.