Birmingham midfielder Damien Johnson will be out of action for up to a month because of a fresh hamstring problem.
Johnson is rated by Bruce as one of his best signings
The Northern Ireland international missed the first six weeks of the season after being injured during a pre-season friendly.
And he tore the other hamstring in a run-out for City's reserves against West Ham on Monday.
It means Johnson will miss Northern Ireland's European Championship qualifier against Sweden on 17 October.
Northern Ireland are already without defender Chris Baird, who is suspended for the Stockholm game, and also have an injury worry about Burnley's Michael Duff.
Birmingham boss Steve Bruce, meanwhile, described Johnson's latest setback as "terrible for him", but backed him to bounce back strongly.
He said: "This kind of thing happens in football and Jonty has to be strong enough to cope.
"He had just got back to fitness and had played his first 90 minutes for nearly six months against Blackburn Rovers in the Carling Cup last week.
"He is unfortunately going through one of those stages, which he's never suffered with before in his career, in having one knock-back after the other.
"The re-occurring injury as a footballer is your worst nightmare especially when its a muscle injury. Kicks, bangs, knocks, cuts, bruises are nothing.
"I've seen Jonty play through all sorts of pain, when his jaw has been hanging off, for example. Talk about John Terry playing on with pain - Jonty is as tough as they come.
"It's a shame because he would have been very important to us with the stage of the season we've got ahead. He would have been ready and willing to take part and do his bit."