Lambert opted to make United States international Guzan his number one this season at the expense of the vastly experienced Shay Given.
But Lambert believes having Given waiting in the wings for a recall has been a key factor in Guzan performing so well.
The Villa boss said: "I think Brad this season has been exceptional (in the Premier League) and I know Shay Given was terrific for us in the cup competitions.
"It's been a big call because there's nothing in it. The two of them are top class goalkeepers.
"Guzan, at this minute and since the start of the season, has been fantastic for us.
"From my own point of view he's been absolutely colossal for the football club. I wouldn't swap him.
"But you've also got to give Shay a lot of credit for what he's done and the way he's kept him in the number one spot really.
"I think some of it (Guzan's form) is down to the fact Shay is pushing him as well.
"He's made some massive saves for us in big moments and he's been a credit to the club. To have a good goalkeeper is vital and fortunately this club has got two top goalkeepers."
Lambert took the big decision to leave out play-maker Charles N'Zogbia and replace him with Barry Bannan for the weekend win at Reading.
He said: "I spoke to Charles the day before the game and explained the situation.
"He's been excellent for us, he really has, but I just thought it was going to be a different type of game from the last two and Barry did well."