Birmingham duo Lee Carsley and Stuart Parnaby have been ruled out of the game against Manchester United on Saturday.
Both players were injured during the FA Cup tie against Nottingham Forest at the weekend and will miss the Premier League clash at St Andrews.
Carsley suffered ankle ligament damage while Parnaby pulled a back muscle in the 0-0 draw, and manager Alex McLeish revealed he is still to learn how long they will be out for.
"Lee and Stuart are both out for Saturday," he said. "We've not put a timescale on when they'll be fit again.
"Lee has got an ankle ligament injury and Stuart has pulled a muscle in his back, so we're not sure how long they'll be sidelined for."
Meanwhile, defender Martin Taylor admits he faces a tough task to get back into the Birmingham first-team.
City have been in excellent form of late in the Premier League and have been watertight at the back so far this term.
The Blues have conceded only 18 goals in the top flight, the joint second-best defensive record in the league, with Roger Johnson and Scott Dann forming a formidable partnership in central defence.
Taylor, who is on the comeback trail after a five-month lay-off with an ankle ligament injury, knows he will have to fight to regain his place.
"The team hasn't changed over the last seven or eight league games because we have been winning and the manager will want to keep that team as much as he can," he said.
"You don't want to change a winning formula. But there may be injuries and suspensions and you may get a chance then.
"I just need to make sure I am ready and keep myself in the right frame of mind to go in when I am needed.
"Before this, I had never really done more than three weeks out. You get small injuries but I had never had anything like this which meant months out.
"It's been a new experience for me. I came back for pre-season and trained with all the lads and then to be out for nearly five months is really hard.
"Any footballer will tell you it's difficult to watch. We have been doing really well, which is fantastic, but you want to be part of it and in the team."