Petrov is now in remission having undergone chemotherapy following his diagnosis in March last year.
Having spent much of the last 13 months fighting his own battle away from the club, Petrov is now able to increase his presence around Villa Park and the club's Bodymoor Heath training base.
He will cheer on his team-mates from the stands when Villa face Manchester United at Old Trafford on Monday and Lambert, who played with Petrov at Celtic, realises the effect his inspirational captain is having.
"First and foremost it's great that he's in remission and as he said, he's going to try to live a normal life," said the Scot.
"I don't think anyone could envisage what he's been through, unless they have themselves. It puts everything in perspective.
"The 19th minute of applause (Petrov's squad number is 19) at the games is fantastic, from home and away fans.
"I've known him since he was 19 when I first played with him and never in a million years did I think anything like that would happen to him, he's such a fit lad.
"Thankfully, he's doing great. He did a little bit of work with the fitness guys the other day. It's great that he's on that road.
"When he comes into the building I'm pretty sure everyone will have a thought in their head, thinking 'my God, he's done that' about what he's been through. You wouldn't be human otherwise.
"It puts things into perspective and the lads love to see him, which is nice."
As for Petrov attending Monday's match, Lambert said: "Whether he's at the game or not, the most important thing is he's still here and he's getting through it."
Petrov will be fraternising with 'the enemy' at the match as he plans to sit with former Villa forward Ashley Young, who joined United from the Birmingham club.
"I'm going to watch the game together with Ashley Young," he told AVTV.
"He wasn't happy, but I said we're going to take something from the game. It will be exciting. The boys are looking forward to the game.
"It's a great stadium, it's Man United and you want to play games like that every week. The boys will be up for it and I hope we can get a result there.
"In today's football you never know. It is so fast, so competitive and we will go there with confidence.
"The players know it's going to be hard because Man United are a class team and they need only a couple of wins to clinch the title.
"We're aware of that, but we will go there with plenty of confidence and courage to win."
Petrov understandably believes Villa can avoid the dreaded drop from the Barclays Premier League.
"There's five games to go, I've been to see the boys this week and they're really up for it," he added.
"They're playing, creating chances and it's great to see.
"Especially from a young team. The young lads have picked up their form and are sticking to it and not giving up.
"Sometimes when you go on a bad run you just give up, but all these young boys have got stuck in."