Grealish has been left out of City's squad for the Club World Cup and the coach has suggested that a fresh start is required for the midfielder after spending the past two seasons on the periphery at the Etihad Stadium.
Guardiola said: "He had a conversation between him and the club and they decided it was best [to stay behind].
"Jack is an exceptional player. The only reason why he didn't play last season is of course my decisions. We decide that he has to play. The club was honest, he was honest.
"We decide the best is to stay [behind] and have a place that he can feel like he can come back to be the player like he was in the year of the treble or all his career in Aston Villa.
"The fact is in the last two seasons he didn't play much minutes. He has to come back to play and have the butterflies in his stomach that he can play every three days, every three days and show again the quality he has."
However, Guardiola said it isn't a certainty that Grealish leaves the club this summer.
The Spanish manager said: "I don't know. Now we decide don't come here and what happens I don't know in the end. If we don't find, he's a player for Man City and he will be back."
Guardiola is joined by Jurgen Klopp's former Liverpool assistant Pep Lijnders in his new-look coaching team and explained that he asked permission from the German coach before approaching Lijnders.
The City boss said: "As always, a long conversation.
"In certain moments I suggested I was thinking about Pep, and he gave me his opinion, that was, of course, so highly was his right hand, one of the most successful periods of Liverpool in the last years, and after I took the time to decide.
"I asked permission to Jurgen, of course, because I didn't want to interfere much. I said to him it would be a problem, you know, to talk to him, and he said absolutely not.
"Pep will help me because at the end, it's a question of back and forth, so his knowledge, I drink from his knowledge, and every day since we met together, and we talk a lot about tactics, about football, about training sessions, and many things, he's been so inspiring for me."
