After beating Liverpool a fortnight ago it was very much a case of 'after the Lord Mayor's show' this weekend as they slumped to a 2-1 defeat at Birmingham.
The Black Cats looked lethargic at St Andrews in squandering the opportunity to climb as high as fourth in the Premier League table.
Although questions have been asked about the size of his ranks following some notable summer departures, Bruce feels losses such as the one at Birmingham can be attributed to the squad not yet being tuned into his way of thinking.
Speaking ahead of Tuesday night's Carling Cup clash against Aston Villa, he said: "I have chosen to go that way with the size of the squad and you just hope you don't get any serious injuries.
"Obviously Lee Cattermole's is a serious injury but I am not going to use that as an excuse. I believe it's a mental thing.
"But we can't forget this club only just survived by the skin of its teeth last season so we can't expect it all to turn around in a few months.
Ups and downs"This club has only been in the Premier League for a little over two years and there are going to be ups and downs.
"You can't play well week in, week out but we look a little bit fragile away from home and we need to do something about it."
Bruce has maintained from the moment he arrived at the Stadium of Light that the Carling Cup is a tournament the club can win and he is likely to put out a strong side against Villa.
He will be without leading scorer Darren Bent though, with the former Tottenham striker sustaining a knee injury at the weekend.
Bruce remains hopeful Bent will be ready for Saturday's visit of West Ham and was relieved it was not more serious.
He said: "He has had a bad knock, it wobbled his knee a bit, gave him a fright.
"It was close to being serious but I don't think it is fortunately. It's not ligaments. He twisted his knee, rattled everything inside."