Pires scored his first goal for the club and was an influential figure before he left the pitch to a standing ovation mid-way through the second half. Ciaran Clark and Nathan Delfouneso were also on target for Villa with substitute Nikola Kalinic netting for Rovers.
Houllier said: "He (Pires) is fitter first of all. He played against Sheffield United in the last round and I think he is getting better and better. He has still got the touch and the eye for the pass and he brings some fluency to the game with his technique.
"I was pleased by the reaction of the bench when he scored. That is something very telling. The whole bench went up and were very happy and proud for him. He is a popular fellow. They like him. He is a nice man.
"We've got to understand he has not got the legs of when he was 30 but he still can run and have an impact."
Houllier will not be appealing against the straight red card handed out to 19-year-old defender Nathan Baker for a challenge on Junior Hoilett after 62 minutes.