The German club have been linked with the midfielder, who has seemingly dropped down the pecking order under new Spurs head coach Tim Sherwood.
Holtby has started just six league games this term and it is nearly a month since his last appearance.
Speculation has suggested Schalke could offer him a route back to the Bundesliga and the promise of regular first-team football.
General manager Horst Heldt has admitted that Holtby is under consideration.
He told BILD: "We consider it (re-signing Lewis Holtby). As well as other options."
However, Heldt intimated that any deal would be reliant on Tottenham agreeing to subsidise the player's wages.
"We will not break our financial framework," he said.
Schalke coach Jens Keller recently admitted that he doubts a deal could be done.