Villa Park is close to sell-out for the return leg with Lambert set to name an attacking line-up in an effort to claw back the two-goal deficit.
"It was the lowest point of our season," said Lambert, "Chelsea was low because of the beating we took.
"I think because of what happened and who we were playing - no disrespect is intended to Bradford - it was really, really low.
"But the whole thing changes now. They've got the lead but they will know themselves that two goals is easily retrievable.
"We're playing at home and we have to take advantage of that. I've got a feeling with the way the stadium will be and the way we are going at the moment that it will be a totally different game."