Spurs last week reached an agreement for the Germany midfielder to move to White Hart Lane when his contract with the Gelsenkirchen club expires in the summer.
But Heldt said that could be brought forward if terms can be reached with Spurs.
Quoted in the Daily Mail, he said: "If Tottenham ask for Holtby in the January window, we will talk to them about it. 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."