It is just over 12 months since Lambert opted to drop the experienced Shay Given in favour of the USA international.
Guzan appeared to be on his way out Villa Park after his contract expired in 2012 but Lambert re-signed the 29-year-old and made him first choice ahead of Given.
Villa's defensive record has seen them go 26 Premier League matches without keeping a clean sheet but Lambert insists Guzan has played a pivotal role for the club.
"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."
Lambert conceded that the fact Guzan was willing to return to Birmingham from the USA to sign told him all he needed to know about the Chicago-born shot-stopper.
"He was away when I came. I think he was in America and he came back," he added. "That speaks testimony to him because he might not have bothered coming back but he gave it a go.
"He had a fair crack at it. He's a top pro and he also has the World Cup next summer to look forward to."