Carew is poised to lead Villa's attack in the absence of Emile Heskey, who will be out of action for a month after undergoing knee surgery. The Norway international, who previously played under Houllier at Lyon, is out of contract next summer.
Houllier said: "John is at the end of his contract and it is an opportunity to express what he wants - whether he wants to stay at the club or whether he wants to go somewhere else. In both cases, I would say he has got to be at his best to put himself in the shop window."
Houllier insisted: "There is no talk of a contract yet. I want to see people. We'll see about it after Christmas.
"It is a big opportunity for John to play a run of games. When he wants to, John can be awesome. I think a run of matches could help."
Houllier is relieved Heskey's setback is only medial ligament damage after the training injury.
He said: "I thought it was a cruciate ligament the way he fell but it's the medial, which is one month."
Another Villa striker, Gabriel Agbonlahor, has returned to light training but will not be available for Saturday's clash at Fulham.
Houllier has blocked England Under-19 forward Nathan Delfouneso's hopes of going on loan for the time being because of the current injuries at the midlands club.