Goalkeeper Stephen Henderson has been ruled out injured for up to two months.
The Nottingham Forest loanee, who arrived back at Pompey for a second stint on transfer deadline day, limped off with a thigh injury in Saturday's 2-2 draw with Doncaster Rovers.
Henderson went down in the 85th minute against Darren Ferguson's outfit and was unable to continue.
As a result, Kal Naismith was forced to turn into a makeshift keeper for the remainder of the game as the Blues had used all three substitutions.
Kenny Jackett revealed the Irishman will be sidelined for at least a month.
The Pompey boss said: 'It's quite severe and he won't be back in a matter of weeks.
'It'll be a month or two I would think, so it's not good news.'