Keller admits he expects to lose their star man at the end of the season.
He said: "I am delighted that we have him on board for the second half of the season but, because of his clause, he can leave when a club puts the money on the table.
"That's why I can't plan with him, because it is just not in my, or our, hands.
"It's a god-awful situation when, as the coach, you don't know whether such an important player will move on, but Julian was injured at that time, so at least any unrest did not necessarily affect the other players.
"He's young, but has made over 100 competitive appearances for Schalke. Others don't achieve that in five, six years.
"That's why he is very experienced already, and has shown his superior qualities often enough, although his youth shows in his consistency.
"It's clear that all the talk about his €45.5m release clause and the interest from top European clubs leaves its mark on such a young lad.
"The hype around him last summer was huge. He must learn to cope with that, and also that failing to perform at less than 100% is not good enough for him."