Keeper Stephen Henderson has been ruled out for the rest of the season.
The Nottingham Forest loanee limped out of his Pompey return with a thigh injury in last Saturday’s 2-2 draw with Doncaster Rovers.
Henderson has since gone back to the City Ground to start his rehabilitation.
It was originally thought the setback would keep him out of action for up to two months.
However, both Blues and Forest medical staff believe it is unlikely the 29-year-old will return to action this campaign.
Pompey boss Kenny Jackett said: ‘He is very unlucky because his availability has been very good this season.
‘He has played in under-23 games, games we have seen and, obviously, John Keeley has kept a close eye on him since he left last time.
‘Unfortunately for him, it just shows you perhaps stepping up into first-team football is slightly different.
‘The estimation from our medical people and Forest is he will do well to play again this season.’
Henderson’s injury will see Luke McGee restored between the posts for the Blues’ trip to MK Dons on Saturday.
The former Spurs man found himself dropped to the bench against Doncaster.
Meanwhile, Alex Bass’ planned loan move to a non-league club has been shelved.
Jackett revealed he will assess Henderson’s situation again when the summer transfer window opens.
But Pompey’s manager’s focus is currently fixed on finishing the current campaign strong.
‘We will have to see, our focus is on this season,’ he added.
‘The summer is a different time, we will look at it then.’
– WILL ROONEY