Mats Hummels
Mats Hummels is favourite to succeed Sebastian Kehl as Borussia Dortmund captainGetty

Manchester United target Mats Hummels is favourite to become the new captain of Borussia Dortmund in what would be a new blow for the Old Trafford club.

United manager Louis van Gaal has admitted the team's defence needs urgent reinforcements, with Hummels at the top of his transfer wishlist.

The German World Cup winner was the subject of a reported £20m bid by United, an offer laughed off by Dortmund manager Jurgen Klopp.

"What are we talking about today? Mats to Manchester United? For how much? €20m [£15.8m]? With those kind of sums, can you forget that straight away," Klopp told a German radio station.

United's recruitment of the 25-year-old now looks even less likely to materialise after the defender emerged as the front runner to assume the captaincy of the Bundesliga club since the armband was vacated by Sebastian Kehl.

Midfielder Kehl, 34, decided to stand down as captain and allow a successor to take charge on the pitch after helping the German giants win three Bundesliga titles since he joined the club in 2002.

Veteran goalkeeper Roman Weidenfeller, 33, has also been touted as a possible candidate to assume the captaincy but it is understood that Hummels stands a better chance of being appointed.

Van Gaal has identified his backline as the most pressing area in need of an overhaul after adding just Luke Shaw to a United defence that lost experienced heads Nemanja Vidic, Rio Ferdinand and Patrice Evra at the end of last season and during the summer.

Speaking last week, the Dutchman said: "I am looking at the defenders we have first. And, when you see our three matches, we are doing well, I think. But it is right that we are looking for defenders, because a lot of defenders have gone.

"We have to replace them but I also like Michael Keane and Tyler Blackett."

Ron Vlaar of Aston Villa and Ajax's Daley Blind have been linked with reunions with ex-Netherlands coach van Gaal but it is Hummels who United saw as the player to build a new defence around.