Pep Guardiola believes Leroy Sane must "move forward" after being left out of Germany's World Cup squad as the omission may "make him stronger".
Sane vowed to return stronger following Joachim Low's decision not to take him to Russia, admitting his disappointment at not being included in the 23-man squad, who were eliminated at the group stages.
Guardiola has challenged last season's PFA Young Player of the Year to prove his worth for both club and country by eclipsing the heights he reached in the previous campaign.
"Leroy has to accept that this decision may make him stronger," said Guardiola, ahead of City's 1-0 pre-season defeat to Borussia Dortmund.
"He's so young and in two years it is another European Championships and then in four years another World Cup.
"I think it's the past now. The national trainer decided what's the best for the national team.
"Leroy has to accept it and move forward. He has the chance to show the national team from Germany and first of all us, he has another season to show his quality."
Guardiola added: "He has to improve his game, try to keep the levels he showed last season. We are so happy to have him here."
Sane played the final 15 minutes of City's International Champions Cup defeat to German club Dortmund in Chicago as they prepare for the new season and their defence of the Premier League title.
"Leroy Sane arrived two days ago," Guardiola added. "We needed some minutes to start and put his head on the field and fight and run. He was ready to play 15-20 minutes but no more than that."