According to the winger's Twitter account, he today underwent a procedure to remove a cyst from his right leg.
Green admitted the issue had caused pain for the '˜last year', prompting him to seek medical assistance.
However, at this stage it is not clear how long the Aston Villa loanee will be sidelined for.
Tweeting from his account, @AndreGreen_, he wrote: '˜After feeling pain in my knee for the last year due to a cyst I finally decided to get it removed.
'˜Thankful everything went well & this small set back can allow me to get back to my best & play 100% fit!
'˜Looking forward to being back with the boys at Portsmouth & get going.'
The injury ruled Green out of last weekend's trip to Peterborough.
The 20-year-old was withdrawn at half-time of his Pompey league debut against Shrewsbury the previous week.