The injury will rule the Scotland international out for the next three months, and Bruce admitted it is a big setback as he felt Gordon was returning to the sort of form which persuaded Sunderland to pay Hearts £9m to sign him.
He said: "Craig gives you that calming influence and he was just getting his confidence back. He played with an injury for a lot of last season and that drained a little bit out of him.
"I was delighted with his form before the injury. He was starting to look the goalkeeper we paid all that money for.
"He certainly caught him. Could he have got out of the road? Only Jermain knows. When I saw it with the naked eye I thought it was 50-50, but when I've looked after . . .
"Craig has a broken arm and he is sore and upset. He has had an operation to put a metal plate inserted and he is disappointed, as you would imagine."