Watzke confirmed on Wednesday that Mkhitaryan was not for sale even though the Armenian playmaker is out of contract in 2017 and rejected an extension offer from Dortmund at the end of May.

"We've been negotiating [a new deal] with Henrikh Mkhitaryan for four months," Watzke told Germany's mass-circulation Bild newspaper. "We now know he won't sign an extension. There was never a promise that he could leave us early."

Mkhitaryan reportedly has his heart set on a move to the English Premier League. But in the wake of the Hummels and Gündogan departures, and amidst international interest in striker Pierre-Emerick Aubameyang, Dortmund are standing firm.

In 2013-14, Dortmund refused a seven-figure transfer fee for Robert Lewandowski, forcing the Polish striker to see out his contract before leaving for Bayern Munich.

Mkhitaryan's agent Mino Raiola said the club was going back on a oral agreement with the player.

"Those in charge at BVB promised Micki months ago that he can leave the club in the summer," Raiola said. "Dortmund have an official offer, but the BVB bosses are afraid of their fans. All of a sudden they say that he has to leave on a free next summer - that's very bad management."

"Manchester United are a unique opportunity," Raiola continued. "Micki wants to join them now by any means."

Mkhitaryan, who joined the club from Shakhtar Donestk in 2013, scored 23 goals and assisted 32 in 52 competitive games last season for Dortmund.