Manager Houllier brought on Emile Heskey and Stewart Downing and both responded with a goal to earn his side a 2-1 success over Burnley at Villa Park after extra-time. Heskey looked to have won the tie in normal time before Clarke Carlisle's last gasp equaliser but then Downing's stunning strike sealed a last eight place.
Houllier said: "I don't take credit for the subs. The manager makes the decision to bring them on - but it's down to them."
He added: "They scored the goals, I didn't. I wanted to rest some players but I knew at some stage, with other players tiring, they would make a difference. Emile made a difference and Stewart made a difference. The problem with the subs is they have got to be in the right frame of mind.
"If they have the right attitude, if the hunger is there, then they bring something. Their freshness can bring initiative which was the case. Emile came on and changed the game because he held the ball up."
Villa ended the game with 10 men after winger Marc Albrighton was red carded for a professional foul on Burnley sub Wade Elliott and he will be suspended for Sunday's derby with Birmingham.
Burnley boss Brian Laws felt his side deserved to go through to the next round.
He said: "Football is cruel at times. We didn't deserve the result.
"The performance was outstanding, with terrific play from us at times. We were very unfortunate not to be in the hat. We were beaten by an outstanding strike and it needed to be as Lee Grant (keeper) was terrific.
"The hero of the night was probably Brad Friedel. He certainly earned his corn with some great saves at the end. All I've got is praise for my players. The performance is a real sign of what we can do."