The condition was diagnosed following tests after the Bulgaria international, 32, developed a fever following last Saturday's defeat to Arsenal. Former England midfielder Thomas was diagnosed with chronic myeloid leukaemia in 2003, but has since recovered and established The Geoff Thomas Foundation to raise funds for a network of national centres.
"You are shocked with any news like this, but with somebody so young and so fit I think it always becomes more shocking," the 47-year-old told Sky Sports News.
"My thoughts go to him and his family. I know exactly where he will be at the moment, it's something totally out of the blue.
"It's a case of taking all the information over the next couple of days and then deciphering everything and then getting on with putting your life in the professionals hands.
"Here in Birmingham we've got tremendous skills in haematology and the doctors are fantastic people so I'm sure he'll be in good, safe hands.
"I think it's more aggressive than the illness I had, it's something that has to be dealt with pretty quickly and it's [a case of] going straight onto chemotherapy and getting into a routine. I'm confident that the guys will look after him and hopefully as soon as possible get him back on the football pitch."
Support quickly flooded in for Petrov following the news on Friday afternoon, and Thomas revealed that similar messages were a big help during his treatment and recovery.
"It was just like being in a dark tunnel, trying to take it all in, but as soon as you start coming round and thinking I am going to take a positive view you start looking for all the positives," Thomas added.
"And the support I was getting from football was a tremendous boost. It was a tremendous lift at one of the darkest times."