Pompey fear Dion Donohue faces a stint on the sidelines after being hit by glandular fever.
The Blues fear the left-back has been struck down by the illness, which will wreck his pre-season plans.
Donohue has not yet featured in pre-season after blisters prevented him playing against Cork City and the win over Hawks.
Now the 24-year-old faces another period out as Pompey continue to build up to the new campaign.
Jackett explained the Welsh man will be assessed over the coming days to decide the best course of action.
He said: 'Dion has suspected glandular fever. It hasn't been diagnosed yet, but it looks as if it could be something along those lines.
'He has a general feeling of weakness, so we suspect that could be where it is.
'It's not a muscle injury or a usual football injury.
'I think we'll see how he goes over the next week and whether that diagnosis is the right one.
'But that is what is suspected, so we have to see how he gets over it.'
Meanwhile, Jackett expects Ben Close to be out for another week with the hamstring issue which is hindering his pre-season progress.
But there are no concerns over Tom Naylor, who hobbled off with six minutes left at Broadhall Way.
Jackett added: 'Ben has the issue with his hamstring.
'It's a tight one and it may keep him out for another week. Tom had a bit of cramp and we didn't want to take any risks with it.'
- JORDAN CROSS