Luke Varney will be sent to see a specialist to get to the bottom of his knee injury.
The Pompey forward’s return from the strained medial ligament he suffered at Burnley in December has taken longer than previously expected.
So the former Blackpool man will undergo another scan and be assessed by a knee specialist to make sure there is nothing serious causing the pain.
Blues boss Michael Appleton said: ‘Luke has been a bit of a frustrating one.
‘He’s due to have another MRI scan as he’s not progressing as well as we’d like.
‘He will see a specialist early next week to double check to see if there is anything on the scans.
‘He’s getting a bit frustrated with himself and he’s not been able to kick a ball without pain, which is disappointing.
‘It’s not much of a setback, it just doesn’t seem to be healing quickly enough or as quickly as we thought.
‘We keep giving him a bit of stick about his pain threshold not being that great.
‘But every time we get to a stage where we hope he will be ready to join in, he doesn’t feel he’s ready to do it because the pain is a little bit too much.
‘We will do all the right things we need to do with him to make sure everything is okay.
‘I daren’t put a time frame on it. I would love to say next week we will get him back training with the boys.’
Aaron Mokoena’s hamstring problem is expected to keep him out of tomorrow’s trip to Peterborough, while David Norris will again be missing through suspension, although he has also undergone a successful double hernia operation.
But Tal Ben Haim should be fit to retain his place in defence, despite missing most of training this week.
Appleton said: ‘Aaron Mokoena felt his hamstring when he upped the pace in training so he will be a doubt again.
‘Tal has missed training for most of this week with a strain near his hip but we’re hopeful he will be okay.
‘Chuck’s (Norris’) op seemed to be fine. He had a double hernia and had the mesh put in.
‘He has been in the gym every morning doing his exercises.
‘Whether he is back for Birmingham or Blackpool, I don’t know but hopefully it will be one of those games.’