It is exactly 12 months since the Bulgarian international became of the illness, before it was announced publicly two days later on March 30.
Petrov is currently in remission and Lambert has nothing but admiration for the way his former Celtic team-mate has tackled the challenge.
He said: "I am always in contact with Stiliyan. He is doing great and I don't think anyone in this room can imagine what he has been going through, the way he has battled so strongly.
"He is doing really well and hopefully it's not too long before he comes out of that stage he's at.
"Stiliyan has been a big, big loss to us. He is doing great. I don't think anyone can comprehend what he has been going through.
I can't imagine what he has gone through at certain times.
"The way he has come through, that has been unbelievable. Lesser people, no wonder they struggle through it.
"He was a really close friend of mine at Celtic and I'm delighted he is getting there."
Villa are at home to Liverpool on Sunday with Lambert convinced striker Luis Suarez is in contention for the PFA player of the year award.
He said: "Suarez must be in the running with Robin van Persie and Gareth Bale.
"Of those three lads, one of them will win it. He's a top player, outstanding."
Villa midfielder Fabian Delph will return after a two-match suspension for incurring 10 bookings this season.
Full-back Matt Lowton is on nine bookings and will have to avoid another yellow card in the next three games to avoid his own two-match suspension.