Manchester United have been put on alert after reports that target Mats Hummels will hand in a transfer request at Borussia Dortmund at the end of the season.
The German defender has been a long-term interest of United, with manager Louis van Gaal an admirer.
Hummels has struggled under new Dortmund manager Thomas Tuchel, and Bild have reported that the player is not happy at the Westfalenstadion.
The German World Cup winner is under contract at Dortmund until 2017.
United's hopes of progressing to the Champions League Last 16 hang in the balance after they drew 0-0 with PSV Eindhoven last night. They must defeat Wolfsburg on 8 December in their final group stage game.
"Anything's possible. We can win anywhere," said Van Gaal.
"We have proved that in the Premier League and we must do it in the Champions League.
"We've made it hard for ourselves ahead of the last game but we'll go to Wolfsburg with hope."
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