Houllier said: "The next step and challenge for Steven is to become world class.
"He has made the leaps from the Academy to the first team, from the first team to the international stage, established himself as an international and now the next step is for him to reach the very top.
"To get there he's got to keep doing what he's good at. He's got to keep working and learning and keep the same attitude.
"Humility, work, attitude and talent takes players to the very top and he has got those attributes.
"He is not there yet. But it is the next level for him and I believe it will gradually come. He is learning all the time, for example, it is not always the long passes with him now, he is varying it well."