Town were sloppy on the ball and all at sea off it as Max Bird’s brace and Conor Washington’s clinical finish saw the Rams cruise into a 3-0 lead with only 31 minutes played, before Nathaniel Mendez-Laing scored a fourth straight after the restart.
Four flattered the Cobblers in one of the most one-sided games that there will be in League One all season, and Brady felt it was as much about Derby playing well than his side being particularly bad.
"What went wrong? Derby are a high level side,” said Brady.
“They’re an incredible team. They move the ball brilliantly and they have great rotations.
"We talked about it before the game and how we have to move when we defend.
"We have to move at the speed of the ball but when you have players who don’t move at the speed of the ball and are leaving big gaps, you see what happens.
"It was plain to see in the first 15 minutes and you saw the players I brought off at half-time.
"We just weren’t at the races and we caught them on a night where they were at their best.
"When they play like that, we need every single player to be an 11 or 12 out of 10 to even have a chance of competing but we weren’t anywhere near those levels.
"They lost at Stevenage at the weekend but it’s a totally different game.
"That was a fight on a tight pitch but on a big pitch like this, you have to shuffle across and make sure there aren’t any gaps and we worked on that but unfortunately they were too good for us.”