Sane, 22, hasn't started a game yet this season and was left out of the Newcastle game completely.
Reports from Goal say that Guardiola is "unhappy with the winger's attitude."
Sane was named Young Player of the Year last season as City won the league but he was left out of Joachim Low's Germany squad for the World Cup.
And during pre-season Guardiola admitted that Sane was "far from his best.”
Without Sane, Guardiola handed a first Premier League start to new signing Riyad Mahrez.