Hans Vanaken was the lucky recipient of Allan McGregor's blunder as he was in the right place at the right time as the former Rangers stopper passed the ball straight to him, allowing the 32-year-old to round him and roll the ball into the net.
"We have learned the first lesson of what we are playing in Europe and that is concentration," Bruce said.
"We made a mistake which cost us the game. Our keeper saved a penalty last Saturday against QPR but today he made a mistake and it's an awful way to lose the match.
"We were very comfortable in the game. We had four good chances to score but we didn't and then he made a mistake in the second half then they got after us much quicker than in the first half."
For the big stories and the banter follow us on Twitter: @tribalfootball
Need a VPN? Or seeking to switch? Then support Tribal Football by clicking here to sign up with our official partners NordVPN. Not only will you receive the best VPN experience, but also great discounts by signing up via Tribal Football. Thanks for your support!