The midfielder has been a star for Dortmund this season following his move from Tianjin Quanjin.
Reports last week claim United maintained an interest in Witsel, but the Belgian insists he's happy at Dortmund.
"Because I felt that Dortmund is the right choice,"he said, according toSport Witness'translation of the Bild report.
"I knew from the beginning that I have the quality to assert myself here and to help. Nevertheless, the BVB was a challenge for me, which I absolutely wanted to accept."