Oli Hawkins is set for a spell on the sidelines after injuring his foot.
The striker-turned-defender limped off during the second half of Tuesday night’s south coast derby defeat at Fratton Park.
It is a frustrating blow for Kenny Jackett, who has been pleased with the 27-year-old’s displays at the back.
The Blues boss said: “Oli had to come off on Tuesday and is going to be out for around four-to-six weeks.
“We wanted to give him more of an opportunity at the back this season after bringing in John Marquis and Ellis Harrison.
"It would be nice to settle the back four down and that’s something we’ve got to keep working away on."
“That’s still the plan and he’s shown promise in that position, but it’s a second injury in what has been a frustrating season so far. We’ve certainly got a good player to come back.
“It would be nice to settle the back four down and that’s something we’ve got to keep working away on.”
Pompey will be able to welcome back two key players for Saturday’s home League One clash with Bolton.
Jackett said: “Ellis and Marcus Harness have both been added to the group after being suspended and injured respectively.
“They’re good additions for us, who will boost the team. It gives us more options on those attacking positions.”