Germany's World Cup winning defender Mats Hummels is already thinking of joining Manchester United, according to Borussia Dortmund manager Jurgen Klopp.

The 26-year-old has long been linked with a move to Old Trafford and is still on United's radar for this summer. Two of Red Devils' former managers, Sir Alex Ferguson and David Moyes made an approach in securing his services.

German legend Franz Beckenbauer and former Liverpool star Dietmar Hamann have urged Hummels to make a switch to Old Trafford. The centre-back earlier admitted that his future plans will be clear in the coming weeks and the Dortmund manager believes his captain is good enough to play for United.

"He's good enough for Manchester United. He's told me he's thinking things over," Klopp told DW Sport.

"We're always looking around the market ourselves. That we want to keep Mats is beyond question. How that works out I don't know. There's always a list of players who would be interesting in the event that you need them."

It is not for the first time Dortmund admitted that Hummels will leave the club in the upcoming transfer window. The Bundesliga outfit's CEO Hans-Joachim Watzke has already revealed the club will be open to let their star player leave at the end of the season.

"In this moment, this a total dummy discussion, because there is no offer. If one arrives, we will discuss it internally and make a decision. But this decision will only be taken by BVB," Watzke explained.

"The situation at Dortmund is like this: We've lost one major player each year, but we don't stop playing football."

"To be clear: Mats knows his contractual situation, he described it in that interview himself. The decision where Mats will play next season will only be taken by BVB," the Dortmund CEO added.

United allowed Nemanja Vidic and Rio Ferdinand leave Old Trafford on a free transfer at the end of the last season. Louis van Gaal signed Marcos Rojo and despite the arrival of the Argentine international, the Dutchman is looking to add an experienced player in the heart of United's back four.