Marcus Harness will be sidelined for another three matches.
But Pompey’s leading scorer could return from his thigh injury in time for the south-coast derby.
The winger has enjoyed an impressive start to his Fratton Park career, netting three goals in his opening seven appearances following a summer switch from Burton.
Harness has subsequently missed the Leasing.com Trophy clash with Crawley – and will also be absent from Saturday’s visit of Norwich under-21s in the same competition.
In addition, according to Kenny Jackett, the 23-year-old will now sit out League One fixtures against Burton (September 17) and Wycombe (September 21).
There remains a chance of Southampton in the Carabao Cup on September 24, however.
Pompey’s boss said: ‘Marcus definitely won't play against Burton or Wycombe, whether he is available for the following week, I don’t know.
‘It’s a very slight thigh strain, it could have been worse, but similarly he will be back training at best next Thursday next week.
‘There is a chance he might make Southampton, we will see.
‘Marcus has been a big player for us and we want him back, he has started his Portsmouth career well.
‘’He has been really good for us, settling into it, his strides game-by-game have been there, which is great to see as well.
‘One way or another, we need him back and are looking forward to him returning to the team, which won’t be too long.’