Warnock and Friedel did not receive many opportunities at Anfield while John Carew was allowed to leave Lyon and join Villa by Houllier in a swap deal with Milan Baros.
Houllier said: "Everybody thinks that when a player leaves a club, it means they are unhappy.
"There was not a problem and there will be no problem with the people here. We can work together.
"Brad, I got on very well with. Don't forget Brad had to leave. Brad came with Roy Evans and had to leave at the end of the season because unfortunately we couldn't renew his work permit.
"That is why I think he went to Turkey and then came back to England. He is a good goalkeeper, very good.
"I'm not surprised at how well he's done because he was a good goalkeeper and that comes with experience.
"As for John Carew, he is an outstanding talent. He is extremely skilful. He can win you a game in an instant. I like the man. It is not a problem."