Petrov was one of the first names on the team sheet under former Villa boss Martin O'Neill but appears set to lose out after Houllier bolstered his midfield in last month's transfer window.
The arrivals of Jean Makoun and Michael Bradley have increased competition in the centre of the park and Houllier said: "Stiliyan is still part of my plans but if he says 'I want to play every game', I'll say 'no'.
"He's the captain so he must lead by example. I'm very pleased with his attitude. He's been influential in me changing the philosophy we have here. He's still got a lot to give.
"You can play only 11 players at one time but it is the squad, and the unity of the squad, that makes you win.
"When there is no competition [for places], teams go down. If you don't accept competition, you don't do this job."
Source: PA
Source: PA