Lambert made a controversial call at the start of the season when he selected the United States international as his number one at the expense of Shay Given.
But Lambert believes that having Given waiting in the wings has been a key factor in Guzan's consistent performances.
"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," said Lambert.
"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."
Meanwhile, Lambert praised Nathan Baker after he put his own goal against Reading behind him to produce a fine performance.
"You've got to give him so much credit. I thought he grew into the game and got better," said Lambert.
Lambert took the big decision to leave out play-maker Charles N'Zogbia and replace him with Barry Bannan for the weekend win at the Madejski Stadium.
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."