The Scotland international suffered the fracture in training last week and will miss the start of the new Premier League season.
Bruce, who is currently with his players at their training camp in Portugal, told the club's official website: "It was devastating - a complete freak accident.
"It happened just when we had Craig at the top of his game and he will be a big miss for us.
"It's a big blow, but we will have to get over it. Every club has its knocks over the season. Craig needs a bit of luck and we wish him well."
Bruce is yet to decide how he will replace Gordon in the short term, although he is confident he has cover in the shape of youngsters Trevor Carson and summer signing Simon Mignolet, as well as the more experienced Marton Fulop, who has been linked with a move away from Wearside this summer.
The manager said: "We have got plenty of competition. Young Simon has come in and Trevor has played in pre-season and done very well, plus there is Marton Fulop, who is with us for the time being."