Drew Talbot is facing a minimum of six weeks in the sidelines.
The right-back suffered a setback when injuring his hamstring in training last Thursday.
Talbot had featured in every league game for Pompey before the incident.
Gareth Evans is to set to continue in that position against Wycombe with Adam Buxton nearing a return to action.
Cool said: ‘It’s six to eight weeks out for Drew.
‘The scan suggests that’s the length of time he’ll be out, which is disappointing.’