Pep Guardiola says it could take up to 10 years for Manchester City to be on the same level as Europe's biggest clubs.
Guardiola failed to reach the semifinals of the Champions League for the first time in his managerial career last season when City were knocked out in the round of 16 by Monaco.
But the club are spending heavily this summer and Guardiola is excited by his future at the Etihad Stadium as he builds a young squad.
"This year we weren't there but you must realise that even Barcelona haven't made it to semifinals each year," he told Catalan newspaper L'Esportiu.
"City are a great club, they have played in the Champions League for five straight years, they are the only Premier League club to have done that. But to reach the level of Barca, Real Madrid, Bayern [Munich], Juventus, that is very complicated. You need time -- a decade."
Guardiola has already added playmaker Bernardo Silva and goalkeeper Ederson Moraes to his squad, with director of football Txiki Begiristain closing in on deals for full-backs Dani Alves, Benjamin Mendy and Kyle Walker.
"Now with Txiki, we are trying to make the changes that are necessary to reinforce the squad. It was one of the oldest on average in the Premier League," Guardiola added. "It was a club which had seen much success, but had grown old.
"We rejuvenated them by adding Gabriel Jesus, [Ilkay] Gundogan, [Leroy] Sane and we need to keep doing that. I have played with eight or nine players of [Manuel] Pellegrini's team. They are good, but we must add more young players.
"However, I have always thought about what might have happened had we had [Vincent] Kompany, Gabriel Jesus and Gundogan [healthy] all season. We might not have won the league, but we would have been in the fight.
"Gabriel Jesus arrived in January and he provided a great spark but he was injured for three months. Kompany came in for seven matches and we won them all. The defence is very important for our passing game."
Guardiola faced criticism for his playing style with some pundits saying it would not work in the more physical Premier League.
But the former Barca and Bayern Munich boss refused to alter his philosophy and is confident it will bring success to City, although he says he must get used to more lenient refereeing than he is used to.
"Everyone says, 'you have to change.' I say, 'you are the ones who need to change.' I am not going to change because I don't know any other way to do this and I don't know how," Guardiola said.
"Here, as with anywhere else, there are 11 against 11 in the same space. The only difference is that the officials allow more and so the matches are closer. Plays that would be called a foul anywhere else, here they are not.
"It is true that there are six clubs who have great economic potential and any one of them can win, but there are smaller clubs who can even the score with direct play, with contact that the refs don't call. But I continue to think that if you play well, you can win."