Mats Hummels could join Bayern Munich in the summer

Borussia Dortmund chief executive Hans-Joachim Watzke says he expects Germany defender Mats Hummels to join Bayern Munich next season.

Watzke is battling to keep Hummels at Borussia and the club captain is one of three stars - alongside Ilkay Gundogan and Henrikh Mkhitaryan - who are out of contract in June 2017.

Gundogan is likely to join Manchester City next season. His agent met City's director of sport Txiki Begiristain in Amsterdam last month, while reports in the media claim a five-year contract has already been signed.

Hummels has admitted he is weighing up an offer, most likely from Bayern, while Mkhitaryan has been linked to Chelsea, although he is the one most likely to stay.

Watzke says he expects to lose some, but not all three of his stars: "I don't believe that we will have all contracts extended and that all three will play here next year," Watzke told Sky Deutschland.

"But it is completely out of the question that all three won't play for Borussia Dortmund next season."

Hummels (left) played for Bayern earlier in his career

Hummels joined Dortmund from Bayern in 2008 after failing to break into the Bavarians' first team. Having already lost Robert Lewandowski to Bayern Munich on a free transfer in 2014, after Mario Gotze had left for Bayern in 2013, Dortmund are eager to avoid seeing Hummels follow suit, but Watzke is realistic.

"I believe that if he [Hummels] goes, it will be in the direction of Munich," said Watzke. "He still has unfinished business with the club. His father, his mother and his brother all live in Munich.

"His girlfriend comes from Munich, he's from Munich too - so a whole lot of components have already come together."

Dortmund chief executive Hans-Joachim Watzke says Hummels could leave in the summer

Watzke says he can understand Hummels' deliberations: "We have to accept when someone like Mats, who has already played for Borussia Dortmund for eight years, is 27 and would come to think: 'Am I going to play for Borussia Dortmund for my entire career, or will I do something else?'"

However, the Borussia chief told Munich-based newspaper Sueddeutsche Zeitung he would "fight for Mat, like I have never before fought for a player."

Watzke said Bayern - or any other club - would have to "dig really deep in their pockets" to sign Hummels, who has a market value of 32 million euros (£25m).

Mario Gotze (left) played under Jurgen Klopp at Dortmund and could return to the club

One Bayern star Watzke would like to see playing in the Black and Yellow shirt again is Germany's World Cup winner Gotze, who left in 2013 for £32m but has failed to make an impact in Munich.

"He came up through the youth teams and he is highly regarded - also by me personally," said Watzke. "If he came on the market and we received a signal that he was interested to come back to us, we'd discuss it."