After getting themselves back into the game, the bees sunk to their second home defeat of the season and continued a poor run of form which has seen them fail to win in their last five league games.
"When the ball goes out for a throw-in or a corner, you have to be looking round and picking up your men. We didn't and we got punished for it." said Rosenior.
"The performance was fine, the work-rate and effort were fine, and some of the passing was good. But if you show that kind of naivety against any side it's going to cost you. In the end all our hard work counted for nothing because of that."
Brentford's next chance to bounce back is next Saturday at home to Bristol City.