He was furious at Posh's failure to hold on to a 2-1 lead as they shipped two late goals to see the Bees leapfrog them in the League One table.
Clayton Donaldson fired home the winner three minutes from time after Peterborough had taken the lead with two second-half goals in as many minutes.
"I can't believe we've lost. When we went ahead I could only see us scoring more," he said.
"I would have been disappointed with a draw so to lose it was a real kick in the teeth. But if you make individual mistakes you get punished and their first and third goals were the result of appalling defending."
He said: "Britt will miss Saturday at Wolves now. I have a three-year-old who behaves better than some of them."
The defeat, courtesy of a Gabriel Zakuani own goal, Will Grigg's bundled second and Donaldson's late winner, extended Posh's losing streak to five games.
Brentford boss Uwe Rosler said: "We showed tempo and quality that belonged at Championship level and we have shown we can compete with two of the favourites for promotion.
"I saw a few tired legs out there because of the intensity of our last two games, but our three substitutes really lifted us and showed that we have a fantastic bench."
Rosler hinted at changes for Saturday's trip to Notts County, but added: "We are a very fit team but our bench is very strong and that is the difference between a mid-table side and one fighting for promotion."
The German admitted his side had got too protective of their one-goal lead, but said they would learn valuable lessons for the future.
He added: "We keep learning but this game was a good lesson for us. When we started playing the ball to feet after the break we got back on it, so that was crucial."
- Sports & Recreation
- Clayton Donaldson
- Britt Assombalonga