The former Manchester United striker ruptured his anterior cruciate ligament during the 1-0 win over Manchester City on Sunday.
The Black Cats completed the signing of Ghana striker Asamoah Gyan from Rennes on transfer deadline day.
“The owners dug deep to make it happen and it became imperative after what happened with Frazier at the weekend,” Bruce told Sky Sports News.
“We can confirm tonight that he has ruptured his cruciate knee ligaments so he’s going to be out for at least six months.
“It’s a tragedy. That’s two we’ve had do their ACL now and after the window’s shut we hope we’re due a bit of luck.
“David Meyler did his in the last game of last season so that’s two in five Premier League games and we don’t need any more of that.
“Fraizer was just starting to show what he’s about but make no mistake, he’ll be back. He’s a larger than life character and he’ll be firing again.”