Birmingham are sweating over the fitness of on-loan Barcelona midfielder Alexander Hleb ahead of Sunday's derby match at Aston Villa.
The former Arsenal player suffered a twisted knee during the second half of Saturday's 2-0 home success over Blackpool.
Hleb had his best game for the club operating in the 'hole' in a new look 4-3-1-2 formation.
Blues boss Alex McLeish said: "At half-time I said to Alex 'give me 15 minutes of magic and we will get you off'.
"It went a bit longer than that and all of a sudden he had a little twist in his knee which we are concerned about.
"We have to wait to see how it is but he is a brave guy. I just hope there is no serious damage.
"He took a pain-killing injection on Saturday morning in his ankle so we will be looking for him to be a quick healer.
"I was going to take him off maybe five or six minutes earlier. I wish I had now."
McLeish said: "We are trying to find the formation that suits the players and that one suited Alex on Saturday.
"It was new for us and we got the victory but it doesn't mean we will use it every week."
McLeish admits he will have to make changes for tomorrow's home Carling Cup tie with Brentford.
He said: "We will have to make enforced changes. One or two will definitely need to rest and players have been playing with injections.
"That has been the case with three or four of them over the last few weeks, and that just can't last for ever because you will burn them out."