"Robert has made his decision. Only he decides when he will make his decision public, though," Kucharski said in a Polish talkshow, according to Goal.com.
"Those who have followed Robert's career will realise that he's never stayed at a club for longer than two seasons. He's already in his third season at Dortmund. Read between the lines and you'll know what this means."
The 24-year-old, who comes to the end of his contract next summer, is thought to have also sparked interest from Juventus.