Ricardo Rocha is set to be sidelined for at least two weeks.
The Portuguese defender hobbled off with a hamstring injury after 25 minutes of Pompey's 3-2 defeat to Hull on Monday.
The problem saw him left out of the side who took on Watford 48 hours earlier.
However, Rocha did take a place on the bench as an unused substitute at Vicarage Road because of the Blues' squad shortage.
Now his hamstring has taken a turn for the worse following his ill-fated appearance against Hull.
Thankfully, the complaint is not as bad as initially feared.
However, boss Steve Cotterill believes the former Spurs defender will miss two weeks at the very least.
And that immediately rules Rocha out of Saturday's FA Cup clash at Brighton and the Championship trip to Nottingham Forest on January 15.
The Blues boss said: 'Ricardo's not pulled it, but it's a hamstring injury.
'He could feel it after five minutes against Hull. We'd been watching it and didn't want him to pull it with a grade-three tear.
'That's the difference between two weeks and eight weeks.'
Along with Rocha, Greg Halford is also missing for Saturday's tie at the Withdean Stadium.
The suspended 26-year-old has returned to Wolves to serve his three-match ban, although Pompey would like him back for the rest of the season.
In addition, Michael Brown and Richard Hughes will continue to sit on the touchline after their contract offers were withdrawn.
The likelihood is neither will play for the club again.
Cotterill has admitted he is likely to resist the urge to start Nadir Ciftci and Tom Kilbey, though.
Ciftci failed to impress during a rare start against Watford, while Kilbey made his debut as a substitute in the same match.
Neither featured in Monday's 3-2 Fratton Park defeat to Hull.
And Pompey's manager remains wary of giving the rookies regular first-team action at this moment in time.
He added: 'Nadir and Tom aren't ready to be playing at this level. Nowhere near it.
'People were raving about Nadir Ciftci earlier in the season.
'I knew where he was because I see him every day. He's still a young boy.
'Tom Kilbey? I don't even know if he will be ready for the level, bless him.
'But they are in environments they maybe shouldn't be in.'