Borussia Dortmund CEO Hans-Joachim Watzke has admitted the club have concerns over the future of Manchester United and Arsenal target Ilkay Gundogan.

Despite missing most of the season through a troublesome back injury the Germany international is widely regarded as one of Dortmund's most important players, providing an influential and industrious presence behind Jurgen Klopp's vibrant attack, attracting the attention of the Premier League champions who are in desperate need of a formidable midfield presence.

The Gunners have also been persistently linked with a move for Gundogan but Dortmund have previously remained fully confident of retaining the services of one of their most prized assets.

However, as the club prepare themselves for the reality of life without Robert Lewandowski - who is set to leave at the end of his contract next summer - Watzke has conceded that the club are unsure whether Gundogan is prepared to commit his long term future to the Bundesliga giants.

"It is our absolute desire that besides Robert Lewandowski no other regular player leaves the club," Watzke told Bild.

"Only in the case of Ilkay Gundogan, we do not yet know how it will turn out. We would love to extend (a contract) with him."

Gundogan is contracted to last season's Champions League runners up until 2015, but is yet to enter talks with his club regarding a new deal.

Talk of a new contract has been spoken of throughout the season by Dortmund, with the midfielder's father Irfan Gundogan stating the player would enter discussions after recovering from his injury, which is expected to rule him out until the new year.

There has been no progress however, leaving Dortmund with the possible prospect of losing two hugely influential players on free transfers or cut price deals in two consecutive years.

Real Madrid have also been heavily linked with a move for the German, with Spanish newspaper Sport suggesting the club already have a deal in place with Dortmund for the midfielder next summer.

Gundogan himself has previously revealed that he would be open to moves to both England and Spain at some point in his career.

"It's not reprehensible to answer such questions (of future moves to Spain or England) with 'yes'," Gundogan said, speaking in April of this year.

"Everyone has a childhood dream on what he would like to achieve one day and my dream always was to play in England or Spain."