Petrov injured ahead of England qualifier
"He has a back strain and we decided it's sensible for him to miss today's training. We'll have more details tomorrow," Bulgaria team doctor Mihail Iliev told Reuters.
Coach Lothar Matthaus added uncapped Levski Sofia winger Simeon Raykov to his squad on Monday in case Petrov, 32, is ruled out of Friday's match at the Vasil Levski national stadium.
England top Group G, ahead of Montenegro on goal difference, with 11 points from five games. Bulgaria are fourth with five points.