Jack Whatmough could be out of action for up to six months.
That is the estimate delivered by Kenny Jackett following a knee operation for the Pompey defender.
Whatmough damaged cartilage in his left knee during training on Wednesday.
The 21-year-old suffered ligament damage to the same knee in March 2015 against Cheltenham.
However, Jackett insists the current set-back is a separate problem.
Now the popular Gosport youngster must stage another fightback following yet more wretched luck with injury.
It’s a shame for him because he started very well for us – and for me – and I have been pleased with himKenny Jackett
Jackett said: ‘Jack has damage to the cartilage in the knee which will keep him out for maybe five to six months.
‘It’s the same knee he injured previously, but a separate injury.
‘It occurred quite innocuously really. He has hyper-extended his leg in training and quite often the worst ones are like that.
‘It’s a shame for him because he started very well for us – and for me – and I have been pleased with him.
‘But he’s young enough to bounce back.
‘It will take him a little while, but he is determined enough and confident enough to get over this.’
Whatmough had been an ever-present in Jackett’s opening seven matches as Pompey’s boss.
He had impressed alongside Christian Burgess in the injury-absence of Matt Clarke.
When Clarke returned against Rotherham, the trio lined up as a back three, although the fixture ended in a Pompey defeat.
Whatmough was substituted at half-time in that 1-0 loss through a tactical reshuffle, with Jackett switching to a flat back four.
The Academy product has recovered from several injury lay-offs over the years to make 57 appearances.
In May he pledged his future to the Blues by signing a new two-year deal.
Now he has targeted a comeback in the new year.
– NEIL ALLEN