Grant Holt's header 20 minutes from time looked to have secured a point for the home side, cancelling out Kieran Richardson's wonderful 20-yard strike midway through the second half.
But Villa fell to a late winner from half-time substitute Hugo Rodallega, who rose to glance home Lewis Holtby's cross and consign the hosts to their third consecutive league defeat, leaving Lambert fuming at his side's defensive frailties.
He said: "It's a poor goal from our point of view. You can't have a ball come into the box without getting touch-tight in the penalty area.
"We made a mistake when (Ashley) Westwood tried to play a pass and to be fair he very rarely gives it away. So that was the first error and the second was not getting touch-tight in the penalty area."
Despite the defeat, though, Lambert insisted that there were some positives to take from the game, including the contribution of on-loan striker Holt, who scored his first goal since arriving at Villa Park from Wigan Athletic in January.
But the Scot admitted that a number of injuries to key players had taken their toll.
He added: "I thought that Grant led the line well for us. I thought he scored a really good goal.
"Matthew (Lowton) played against Manchester United last week and did fine. We lost Fabian (Delph) this morning, which was disappointing. Gabby (Gabriel Agbonlahor) handn't recovered from a bug.
"When you throw in (Karim) El Ahmadi, that's a big chunk of our team because they've been ever-present really.
"But it gave some of the other lads an opportunity and to be fair to them they came in and did quite well."
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