Manchester United are concerned Ilkay Gundogan’s decision to leave Borussia Dortmund could scupper their move for Mats Hummels.

United have been linked with the German international pair as Louis van Gaal looks to build a squad capable of challenging for the Premier League title.

Borussia Dortmund sporting director Michael Zorc announced over the weekend that Gundogan, who is also reportedly interesting Bayern Munich, is set to leave at the end of the season.

But United are aware of Dortmund’s long-standing policy of only allowing one star player to leave each summer.

And there is growing concern that Gundogan’s decision could stop long-term target Hummels from moving to Old Trafford during the next transfer window.

There is, however, hope that Jurgen Klopp’s departure could prompt the German club to relax their stance in order to give incoming coach Thomas Tuchel more room to reshape a squad that is currently ninth in the Bundesliga with three games remaining.

Gundogan will be out of contract at the end of next season and is refusing to sign an extension.

And he looks set to leave this summer rather than move for free at the end of his current deal.

Hummels, who is under contract until 2017, is yet to announce where he will play next season.

Van Gaal was keen to sign the 26-year-old centre-back last summer, only for Dortmund to knock back his interest having already lost Robert Lewandowski to Bayern Munich.

In 2013, Mario Gotze made the same move to the Allianz Arena.

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Click through the gallery below to see Manchester United's summer transfer targets in pictures.