Campbell has not played for Sunderland since August after sustaining a cruciate ligament injury, and Bruce said this week that the striker was almost certain to miss the run-in for the current campaign.
However, it now appears the problem is even worse than feared and could keep him out for much of next season if he has to undergo further surgery.
"We do fear for him," he said. "We think the surgeon needs to go and have a look inside his knee again and, if it does need repairing again, then Fraizer is going to miss the best part of next season as well, which is an even worse blow for us and, more importantly, for the kid.
"Nobody could have done his rehabilitation better than he did, but he fell over and twisted it on the Friday before we played Manchester City.
"The way it's going for us at the moment, we do fear that the boy might need another operation, and that could rule him out for the best part of next season as well."
Bruce has at least been boosted by on-loan Danny Welbeck's return from a knee injury for the trip to Birmingham on Saturday as Sunderland look to reverse their slump in form.