Borussia Dortmund admitted on Wednesday the season is over for Germany's fallen star Mario Goetze as the World Cup winner continues his battle against an unspecified metabolism disorder.

The world appeared to be at the gifted Goetze's feet in 2014 when his extra-time goal won the World Cup final at Rio de Janeiro's iconic Maracana stadium.

But his career has gradually gone off the boil since then, culminating in the recent revelation he has metabolism disorder.

"Provided his treatment goes positively, Goetze can start again with performance-orientated training in early summer," Dortmund said.

"The overall sporting goal for the player is to be fully operational for the start of next season," which starts on Friday August 18 in Germany.

The Dortmund website said: "good news for Mario Goetze: Borussia Dortmund's Germany international speaks positively about the first phase of his treatment," but included no direct quotes.

Last month, Dortmund, who have reached the Champions League's quarter-finals, revealed tests to see why Goetze had been suffering from repeated injury and a lack of form had shown up 'metabolic disturbances'.

Goetze, who has won 62 caps for Germany, was then sent for specialist help and Borussia's CEO Hans-Joachim Watzke has said it would not be a "short-term issue".

Last July, Dortmund paid 22 million euros ($23m) to buy Goetze back from Bayern Munich, having been forced to sell him in 2013 for 37 million euros after the Bavarians triggered a release clause.

However, poor form and injury meant he has played just six games over the 90 minutes this season and he has been a shadow of the player whO left Dortmund in 2013.

Meanwhile, Dortmund announced on Wednesday they have extended the deal of midfielder Gonzalo Castro, 29, until 2020.

Castro is the third first-team player Dortmund have locked into a long term contract recently after Julian Weigl (until 2021) and US teenage international Christian Pulisic (2020).

The club has also told Germany's bad-boy World Cup winner Kevin Grosskreutz, who hails from Dortmund, he can train with Borussia's reserve team after he was sacked in disgrace by second division VfB Stuttgart.

"If he asks us, our answer will definitely be 'yes'. Kevin will always have a place with us and is part of the Borussia Dortmund family," said the club in a statement.

Grosskreutz was part of the Germany squad which won the 2014 World Cup, but Stuttgart sacked him after he reportedly took three youth team players with him on a boozy night out, which ended in a brawl.

He won the Bundesliga twice during a succesful six-year stint with Dortmund, who he left in 2015, and also played in their 2013 Champions League final defeat to Bayern Munich at Wembley.