The American has failed to keep a clean sheet in almost a year since he became Villa Park number one in place of Shay Given but Lambert is still a massive fan.
"He's been brilliant, but leaving out Shay is one of the biggest decisions I've had to make as he's a brilliant goalkeeper and a really good guy," he said.
"But I thought I'd give Brad a go and he has never looked back.
"If anything he has got even better with more confidence. Everything about him has changed."
In the past 12 months, Guzan has not been able to complete a shutout in 26 Premier League matches.