Borussia Dortmund's CEO Hans-Joachim Watzke claims they beat Bayern Munich to sign French starlet Ousmane Dembele, but admits it will be harder to halt the Bavarians bid for a fifth straight German league title.

The new German league season starts on Friday with Bayern, under new head coach Carlo Ancelotti, hosting Werder Bremen at Munich's Allianz Arena while Dortmund start their Bundesliga season on Saturday at home to Mainz.

Winger Dembele was voted France's Young Player of the Year last season and Dortmund paid Stade Rennes 15 million euros ($16.9m) in the off season, beating Bayern to the 19-year-old's signature.

"At that moment, we were quicker (than Bayern), but I don't know how deep their interest was," Watzke told Sport1.

"We got to him early and discovered him before he had played in the (French) league.

"He is an exceptional talent, but there is more to it than that, he has to learn the language and assimilate."

The France Under-21 international only made his French league debut for Rennes last season, but he announced himself with a superb goal for Dortmund in their 4-1 win over Manchester United during last month's tour of China.

Bayern have won the German league title for each of the last four seasons since Dortmund took the 2011 and 2012 titles.

But having signed Dortmund captain Mats Hummels and Portugal midfielder Renato Sanches, the Bavarians are again clear favourites for the 2016/17 league crown with Ancelotti having replaced Pep Guardiola as head coach.

"They (Bayern) have established themselves on the same level as Real Madrid, so it would be rubbish to say we'll attack Bayern," said Watzke.

"You can only attack when the opponent has a weak period."

Dortmund have made eight new signings, including Dembele plus Germany forwards Andre Schuerrle and Mario Goetze.

Germany's World Cup winner is back in the black and yellow shirt after three frustrating years at Bayern, when the World Cup winner was either injured or left on the bench by ex-Munich coach Pep Guardiola.

Watzke says he believes Goetze, who came up through Dortmund's youth academy, went to Bayern for the wrong reasons, especially as Guardiola had wanted to sign Brazil star Neymar, but eventually brought Barcelona's Thiago Alcantara to Munich.

"I believe the problem actually was that he (Goetze) wanted to work with Pep Guardiola more than to the club (Bayern Munich) itself," said Watzke.

"Then he realised that Guardiola had wanted to sign Neymar, that was where the misunderstanding lay."