The Germany international was reported to be the subject of interest from Premier League side Arsenal in the January transfer window, but the 20-year-old remained at the Veltins Arena.
Draxler, who is closing in on a return from an ankle injury, has a release clause of €45.5 million in his contract an Keller knows the talented youngster could move on at the end of the season.
"I am delighted that we have him on board for the second half of the season but, because of his (release) clause, he can leave when a club puts the money on the table," Keller told Sport Bild.
"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."
Keller's side have enjoyed a positive campaign, sitting fourth in the Bundesliga while also having qualified for the last 16 of the UEFA Champions League.
Despite the return of Klaas-Jan Huntelaar and regular contributions from the likes of Kevin-Prince Boateng and Jefferson Farfan, Schalke had lined up a replacement for Draxler.
"It would have been bad if we didn't have (a replacement). When you have to replace a player like Julian, you had better be prepared," Keller added.
The Schalke coach also believes it is only natural for Draxler to be affected by the extra scrutinty he is under due to the price tag place on him.
"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." he stated.
"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 per cent is not good enough for him."
