"I spoke with him and he knows the reason why", Guardiola said of the decision to leave him back in Manchester.
Captain Vincent Kompany will miss out for City once more due to his latest calf injury setback.
In the first part of the last season, Toure was involved in a stand-off with Guardiola, and only when he apologized to his manager, he was given a place in the City squad.
Guardiola, speaking ahead of the game at Feyenoord Stadion, insisted it was a "sporting decision" rather than anything to do with training, but left a distinct impression that not all was well.
Toure has endured a hard relationship with Guardiola since the Spaniard took charge at the Etihad at the start of last season. Six players finished their contracts last season and Yaya is the only player who we decided could stay because he is a special player. Hopefully he'll play soon but it depends on him'.
"All of them (City's players) are happy but I demand as much as possible, I always want more, not just from Yaya, but from them all".
A final decision over his fitness will be made on Wednesday but Guardiola revealed that City were testing him with similar situations to see whether his confidence had been affected by the incident.
"He just wants to be around the team and be in that environment".
"I would understand that [nervous] feeling, but in today's training session there were two or three long balls and he was there".
Toure was excluded from the City squad for the first months of Pep Guardiola's reign past year, firstly for refusing to leave the club on a transfer last summer and then for comments made by the Ivorian's controversial agent, Dimitri Seluk. His face - everyone has seen the stitches and swelling.
"If you are expecting me to say I am better than Feyenoord, I am not going to say that", he added. "We are Manchester City, we have to win our respect in Europe, and every game in this competition is a chance to do that".