Sane was a key member of the Citizens' Premier League-winning campaign last season and was also named PFA Young Player of the Year.
However, the 22-year-old is yet to start a league game this season and was left out of the squad that defeated Newcastle over the weekend.
"Sometimes you have the feeling with Leroy's body language that it is all the same whether we win or lose. He has to improve his body language," said Kroos, according to SkySports.
"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.
"What is crystal clear is his 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.
"If he performs, he's a real weapon. He has the quality, especially for us, but maybe the head coach looked at his performances for the national team and wasn't happy."