The midfielder was linked with a move to Arsenal in January, though the German club rejected all approaches for the 20-year-old.
"We cannot keep a super talent like Julian forever," Tonnies is quoted as saying by Goal.com. "It also wouldn't be fair to refuse players to jump on the train that is waiting for them.
"We are often talking. I told him he should stay with us another two, three years and that a move would be too early for him now."
Draxler has made 89 appearances for Schalke, scoring 14 times.