The Bees suffered their first defeat since the opening day of the season after crashing 3-0 at Chester City on Tuesday night.
There was little consolation to be had from their first defeat in 13 league matches but boss Andy Scott said it was a warning that his players could not afford to let their standards drop and that he would be looking for a quick turnaround on Saturday against Rochdale.
"We didn't do the things we have been doing and it might be a kick up the backside a few needed," Scott said.
"We need to perform better and we need to get back to our ruthless best."
The Bees trailed to a first half goal from Ryan Lowe, but it was two goals around the hour mark that sealed their fate - Kevin Roberts adding the second before Lowe grabbed his second.
It was Brentford's first defeat at Chester in seven visits and they drop down to fourth in League Two as a result.