Adam McGurk faces a month on the sidelines after his pre-match injury at Bristol Rovers.
The former Burton Albion striker was set to start at the Memorial Stadium but sprained an ankle in the warm-up.
That saw Jayden Stockley drafted in as a last-minute replacement.
McGurk will now miss the home clash with Exeter City (Tuesday, 7.45pm) - and is now expected to be out for several weeks.
Boss Paul Cook said: ‘He just went over on his ankle and he has sprained it.
‘I was in having a cup of tea when it happened.
It looks like he will be out for three or four weeks so it’s a bad onePaul Cook
‘It looks like he will be out for three or four weeks so it’s a bad one.
‘It is disappointing as we were just getting a run of games into him.
‘He was just starting to get a bit of momentum but, unfortunately, he is going to be back to square one.
‘But we will have another string to our bow when he comes back fit.’
Although Pompey skipper Michael Doyle is available for selection again, he may have to settle for a place on the bench with Danny Hollands and Adam Barton performing well at Bristol Rovers.
And Cook has yet to decide whether Gary Roberts is fully fit to start against the Grecians.
Cook added: ‘Gary Roberts is just being niggled by this ankle injury, which is just not right. We are nursing him through sessions and into games, if we can.
‘Michael Doyle comes back into the squad and Nigel Atangana trained and looks okay.
‘But Adam and Danny did very well and sometimes people have to take their medicine and wait for their chance again.’