Villa were strong favourites to book a place in the Wembley final but it was the lower-league Bantams who progressed 4-3 on aggregate, with a 2-1 second-leg victory at Villa Park not enough to prevent a heartbreaking exit.
The midlands club start their assault on this year's competition on Wednesday night as they entertain Sky Bet League One side Rotherham.
And Lambert - who took the reins last summer - insists Villa are a different club seven months on from that semi-final upset, boasting new players and a fresh mindset, but that does not erase the memories.
Asked if it was a low point personally, the Scot replied: "Oh for heaven's sake yes, really, because the results in the league weren't even going for us either, so you had a double hit. But all credit to the team they stuck with it, got together and did great.
"There's no doubt whatsoever (it was the lowest point). The two games were low, even though after the first one we still had a chance in the second one. It was still low to lose it. I never blame the team."
Villa recovered from the hurtful exit to win six of their final 13 Barclays Premier League games at the back end of last season and avoid relegation from the top flight.
They do, indeed, appear a different animal this term having shown plenty of encouraging signs amid an opening-day victory at Arsenal and narrow defeats to Chelsea and Liverpool.
And for Lambert, Wednesday's tie against Rotherham is not about avenging last season or unfinished business.
"Last year's totally gone," he said. "It's a different feeling at the football club, you can sense it. It's different to what it was last year. There's different players. Some of them are the same lads but there's a different mindset there to what it was.
"Last year's gone, as disappointing as it was, but it's finished."
Villa's first-choice goalkeeper Brad Guzan, who was rested to the bench for cup competitions last term, echoes the thoughts of his manager.
"It's in the past. That was last season," the United States international said.
"For us we've got a new group of guys and it's about looking forward. We approach each game with the right mentality and focus and hopefully Wednesday we can get a win and put ourselves into the next round."