Brett Pitman has undergone a keyhole procedure to remove a piece of floating tissue in his knee.
The striker had been feeling discomfort for the past month and a decision was taken to operate now as it was becoming more irritable during training and matches.
Pitman has also had an operation to set the broken nose he suffered against Fleetwood in September.
Pompey’s skipper is expected to be out for between 2-4 weeks, depending on how quickly his knee settles.