Brandon Haunstrup has been ruled out for the next few weeks with a knee injury.
The Pompey left-back felt pain in the Boxing Day victory against AFC Wimbledon and aggravated the problem during training.
After having a scan, it emerged the academy graduate has a slight tear in his patellar tendon, which forced him to miss the 3-1 win over Northampton.
And Haunstrup is expected to be on the sidelines for a period of time.
Blues assistant manager Joe Gallen said: ‘Brandon has hurt his knee.
‘In the last couple of minutes against Wimbledon, he turned quick and twisted and felt a little bit of a pain.
‘He then came out to train two days later, completed the warm-up and then went in and went for a scan.
‘We didn’t really think too much of it but, ultimately, there’s a bit of a tear in his patellar tendon.
‘So I envisage Brandon to be out for a couple of weeks. We’re really disappointed about that, it’s not something we expected.
‘In training, he did most of the work everyone else did and the physio surprised us by saying he’s gone to get a scan.
‘We didn’t think a lot of it but, unfortunately, Brandon will be out for a little while.’
Danny Rose was forced off with a suspected fractured leg after picking up the injury in a drop-ball situation with the Cobblers’ John-Joe O’Toole.
Gareth Evans also limped out of the game with a hamstring setback.
Pompey’s vice skipper is likely to be absent for the New Year’s Day trip to Bristol Rovers.
Gallen added: ‘Gareth has pulled his hamstring. It’s the old playing three games in a short space of time caught up with him.
‘It depends on the grading system of a hamstring.
‘Grade one is two weeks and grade two is four-to-six weeks etc.’