Holtby confirmed on Friday that he has chosen Tottenham and will join them when his contract runs out in the summer.
But the move could be brought forward after Schalke admitted they were ready to discuss selling him now.
"If Tottenham ask for Holtby in the January window, we will talk to them about it," said managing director Horst Heldt. "Nothing has happened yet, but, from our point of view, we are prepared to speak to them.
"It would have to be done quickly, because we would have to find a replacement for him."