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But now, scans have revealed the Blues left-back has ruptured his patella tendon and will require surgery.
Bruce said: "Martin was just doing some running in a straight line as part of his rehabilitation work when the injury happened.
"He obviously had a weakness there and it decided to go. It is a big blow to us.
"We have got a few injuries at the moment and most of them are to defenders," said Bruce.
"We just have to hope that some of the players return from injury in the next couple of weeks but we certainly don't want anyone else out."
With Grainger missing Bruce is left with no cover in the left-back position - natural stand-in Michael Johnson is currently out injured and youngster Tom Williams is out on loan for the season at QPR.
My feeling is he'll probably draft in Curtis Woodhouse to fill the left-back spot when Blues take on Newcastle on Saturday evening.
Woodhouse has played at left-back before for Bruce when they were both at Sheffield Utd. and he has recently played two reserve games in that position.