David Wagner suffered the heaviest of his four defeats as Huddersfield Town head coach and admitted: "We just weren't good enough in the first half."
Town trailed by three at the break before an improved performance in the second half, when it took a controversial penalty for Brentford to score.
"The first half was the worst 45 minutes we have played together," said the German, whose side are 19th in the Championship, three points above the relegation zone.
"We had no control over the game, we weren't able to keep the ball. Our first touch was bad and we were bad in passing, so the pressure from our opponents got higher and higher.
"The biggest problem was we didn't trust enough in ourselves and our game and at half-time I asked the players to start again and show some character.
"We couldn't change the first half, but we could do something good in the second half and I thought we did that. We had very good chances and scored two goals."
Wagner had to do without midfielder Jonathan Hogg because of a hip injury and said it posed him real difficulties.
He paired Duane Holmes with Martin Cranie in the first half, and after replacing Holmes with Kyle Dempsey at half-time, played Jamie Paterson in a deeper role.
"We were already without two starting players in Dean Whitehead and Emyr Huws in this position, and Philip Billing is also out through injury," he explained.
"To lose Hoggy as well was another problem."