The midfielder left the Gelsenkirchen club for Tottenham in January of last year but has failed to become a first-choice regular at White Hart Lane.
Schalke have indicated they would be happy for him to return to the Bundesliga but not until Jermaine Jones leaves the club.
Keller said: "We can't really imagine such a transfer financially. Talks have not yet reached an advanced stage.
"Lewis has already played as a defensive midfielder under me. He can play there and is tactically very well-schooled."
Schalke will face Real Madrid in the last 16 of the UEFA Champions League next month.