Birmingham midfielder Sebastian Larsson will undergo a scan in the next 24 hours to determine the extent of his latest medial knee ligament injury.
But Blues boss Alex McLeish is optimistic the Sweden international and ankle injury victim Mikael Forssell could be available for Sunday's derby at Aston Villa.
Larsson limped out of the action after 23 minutes of Saturday's 1-1 home draw with Everton after picking up a problem in a challenge with Leighton Baines.
He suffered a similar blow against Newcastle last month but recovered within five days to face Reading.
McLeish said: 'Seb has tweaked his medial ligament. He is a good healer, as he showed when he came back so quickly last time.
'The physios are not overly concerned and we will see how Seb is when he comes back in after the weekend, when he will probably have a scan.'
Larsson said: 'It wasn't down to any real contact with him (Baines). I've gone to block his shot and it has opened up exactly the same thing I had a few weeks ago.
'It is my knee. It is the same as I did against Newcastle. I don't really know about the derby at the moment. I am having a scan again. Obviously, it is the second time in a short period I've done this.
'I'm really hopeful. The last time I recovered in time for the next game and that is what I am desperate to do.'
Forssell had to be substituted at half-time against the Toffees and was replaced by Garry O'Connor.
McLeish said: 'Mikael turned his ankle. It was up like a balloon at half-time. There was no way he could play in the second half but the physios are optimistic about his recovery for next week.
'The extra day, with the derby being on the Sunday, will probably help.'
On a brighter note for McLeish, striker Olivier Kapo should be available to face Villa after nearly two months out of action with the hamstring problem he suffered against Arsenal in February.
McLeish said: 'Kapo should be available to us next week. He has been out eight weeks now and I am sure his hamstring should be strong enough to push for a place in the squad.'