The attacking midfielder was heavily linked with a move to Arsenal during the January transfer window but the proposed move did not materialize.
However, Keller has conceded that Draxler could easily depart Gelsenkirchen in the summer.
"I am very happy that Julian is still at Schalke for the second half of the season. [But] we cannot stop him from leaving when a club meets his buyout clause," Keller told Sport Bild. "That makes it's hard for me to plan with him, because his future is not in Schalke's hands.
"The Draxler situation in January was unnerving. It's ridiculous when you don't know as a coach whether such an important player will leave or stay. The unrest didn't really affect the team, though, as he was injured at the time."
The 20-year-old, who has a contract with Schalke until June 2018, has netted five goals in 24 appearances in all competitions for his club this season.