Pompey are assessing the extent of Jack Whatmough’s hamstring injury.
The central defender was forced off in the 34th minute against Bristol City on Saturday, replaced by Tom Davies.
In his absence, the Blues secured an encouraging goalless draw at the Championship side.
Whatmough had earlier fouled Joe Bryan, prompting him to receive treatment, but was able to continue.
However, he was taken off as a precaution minutes later.
Paul Cook insisted the problem was unconnected with the knee injury which previously kept him out for 10 months.
Now Pompey’s medical department are waiting to learn the full extent of the issue.
Cook said: ‘It looks like Jack is feeling his hammy.
‘It is nothing to do with his old knee injury, not at all. Fingers crossed for him.
‘Him coming off was precautionary, we’ll keep assessing it and will know more soon.’
In the injury absence of Matt Clarke, Whatmough had established himself alongside Christian Burgess during pre-season.
He had made five starts as he continues to re-establish himself in the side following the frustrations of the past year.
Meanwhile, Cook has explained the Ashton Gate absence of Paul Jones.
The 30-year-old has lost his place following the arrival of David Forde on a season-long loan from Millwall.
However, he was not present in the squad for Saturday, with Alex Bass instead serving as the substitute keeper.
Cook added: ‘Jones is feeling his thigh again, there’s nothing we can do about that.
‘If he feels his thigh then he can’t play.’
– NEIL ALLEN