The Premier League side were well beaten by their League Two opponents in the first leg of their Capital One Cup semi-final on Tuesday night at Valley Parade.
Goals from Naki Wells, Rory McArdle and Carl McHugh - all of them from set-pieces - leave Villa up against it despite Andreas Weimann's goal and Lambert could not hide his disappointment.
"I'm really disappointed we lost the game," Lambert told Sky Sports. "We are in a game now, that's for sure.
"We knew what was coming - set-pieces - and we never defended them well at all. That's definitely what's hurt us but that's certainly not good enough.
"We had a lot of the game and yes they were set-pieces but set-pieces are a vital part of the game and you've got to defend them. We didn't do that.
"But it's half-time and we've got time to retrieve this. We'll see what happens in a fortnight's time.
"We know what we've got to do and we've certainly got to do a lot better than that."