Sane, left out of Germany's squad for their World Cup campaign in Russia, was recalled by manager Joachim Loew for the Nations League encounter against France on Thursday and Kroos suggested the 22-year-old had to show more desire.
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"Sometimes you have the feeling with Leroy's body language that it doesn’t matter to him whether we win or lose. He has to improve his body language," Real Madrid midfielder Kroos told reporters.
He's a player who has everything you need to be a world-class player but sometimes you have to tell him he has to perform better.
Sane scored 10 goals in 32 league appearances last season as City cruised to the title but he has not been able to force his way into Pep Guardiola's starting side in this campaign.
Sane was left out of the squad for Saturday's 2-1 victory over Newcastle United but Guardiola denied reports that the German's omission was due to an attitude issue.
"What is crystal clear is Sane's quality, his pace, his left foot. He was fantastic for City last year but Pep has the same problem at the moment -- he's trying to get the best out of him," Kroos added.
"If he performs, he's a real weapon."