Alex Bass will spend up to six weeks on the sidelines.
The Pompey keeper has picked up a knee injury while on loan at Torquay United.
Bass collected the setback when he was caught wrong-footed from a deflected shot during the Gulls’ 2-2 National League South draw with Woking last month.
It caused the Blues academy graduate’s knee to swell up and, after having a scan, he’s been ruled out of action for more than a month.
Bass has returned to Fratton Park to undergo his rehabilitation.