Warren Gatland's men went into the Canberra clash on the back of five convincing victories but they were caught cold against the Super Rugby pace-setters, who held on for a 14-12 win
Best was bitterly disappointed by the performance but he insists the Lions cannot dwell on the defeat and must quickly refocus on Saturday's first Test against Australia.
"We went in fresh, had a good team run and felt very confident but we knew it would be tough," said Best.
"We just didn't turn up right from the off, they wanted it more and took their chances.
"We let our standards drop and it's up to the 15 and 23 picked on Saturday to reassert that.
"We didn't want to lost momentum but we did that. We now have to face the heat, dust ourselves down and see where we are."
The Lions handed debuts to three emergency recruits in Billy Twelvetrees, Brad Barritt and Christian Wade, while Welsh legend Shane Williams made a one-off start after answering another injury SOS.
But Best is adamant that the unfamiliarity and in some cases a lack of preparation was not an excuse. And he cited the breakdown and malfunctioning set-piece as the cause of the defeat.
"We got physically beaten up up-front, they had a lot of pressure on our set-piece and we got a bit nervy," added Best.
"We won't look for excuses, we brought in quality players, we had a quality team and just got beaten up at the breakdown.
"We definitely got nervy and simple little units miss-fired. At this level if you can't secure ball you will always be under pressure.
"As forwards we will stand up and take a lot of the heat, and deservedly so.
"But credit to the Brumbies they were hungrier than us right across the board."