The 34-year-old signed a one-year contract at the Etihad Stadium in the summer, but is yet to feature for the first team this season.
In addition, the Ivory Coast international is also not in Guardiola's squad for their opening Group F game away at Feyenoord on Wednesday.
Pressed on why Toure has stayed behind in Manchester, the Spaniard told reporters at Tuesday's press conference: "I spoke with him, he knows the reason why. But it's between me and him.
"It's just a sporting decision. Yaya was so important last season. Six players finished contracts last season, the only one who stayed is Yaya, because he is a special player.
"But the decision why he hasn't played this season and is not in the squad - he knows the reason why."
Last season, a spat between Guardiola and Toure's agent Dimitri Seluk led to the player being frozen out of the City squad for several weeks.