Kenny Jackett confessed he was surprised when he learnt loanee Andre Green required surgery on his knee.
The winger, on loan from Aston Villa, had a cyst removed from his ACL this week.
The minor procedure on his right knee is likely to keep the 20-year-old out for a month and could rule him out for at least six Blues games.
Green only arrived at Pompey at the end of August, signing on loan for the remainder of the season, following five Championship appearances for the Villians this term.
The player underwent a medical as part of the transfer formalities and went on to make his Blues debut in the Checkatrade Trophy win against Gillingham.
However, according to Jackett, the cyst didn’t show up during the ‘extensive medical’.
And that caught the Pompey boss cold when informed Green required an operation.
Jacket said: 'Andre Green had a cyst removed from his ACL.
‘Nothing showed in a very extensive medical for us, so we were surprised when he came in and wanted the cyst removed because neither the player or Aston Villa saw it as a problem going forward or flagged it up.
‘But it has obviously flared and put him in a position where he felt it was best where he had it removed.
‘In those circumstances, and although I was genuinely surprised by his request, people don’t have operations or have operations lightly, so we accepted his decision and what we have to do now is back him and look forward to welcoming him back into the group when he is fit.’
Using his Twitter account, Green admitted the cyst had caused pain for the ‘last year’, prompting him to seek medical assistance.
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!’