Oli Hawkins is set to be out for up to six weeks after picking up an injury in the south coast derby.
The Pompey striker turned centre-half was forced off in the 59th minute of the Carabao Cup defeat at Fratton Park on Tuesday night.
Hawkins was replaced by Paul Downing when the score was 2-0, Southampton then grabbed two late goals to make it 4-0.
Following the match, the recently converted centre-half left the ground in a protective boot.
Speaking in his press conference this afternoon, Pompey boss Kenny Jackett revealed Hawkins is set for a spell on the sidelines.
Jackett said: ‘Oli came off, he’ll be four to six weeks.
‘Other than that we are all okay from Tuesday night and both Ellis Harrison after suspension and Marcus Harness after a muscle injury will both be available and boost the squad.’
Hawkins struggled with a back injury in pre-season.
He will miss Saturday’s home game against League One strugglers Bolton.
Looking ahead to the game, Jackett added: ‘Their results have improved obviously with the fact that they’ve taken quite a number of players in as well towards the end of the last window when they’ve had a takeover.
‘They’ll get all the respect that everybody deserves and we’ll be working hard at working out their key strengths and key weaknesses as we do every week.
‘After that we want to keep improving ourselves. I wasn’t happy, we weren’t happy with last Saturday at Wycombe it was a 50/50 game but similarly we didn’t get it right and our performance wasn’t right.’