The 23-year-old has struggled to make an impact since joining Spurs a year ago and his agent Marcus Noack has suggested a return to the Royal Blues in a bid to boost his dwindling hopes of appearing at the World Cup in the summer.
"Lewis is desperate to play, which is why we are looking intensively for solutions," Noack told the Bild newspaper.
"Schalke is an interesting option."
With Jermaine Jones requesting a transfer, Schalke could find themselves short of numbers in midfield and a loan of Holtby until the end of the season could be an option.
Holtby left Schalke for Tottenham a year ago after 18 months with the Bundesliga club.