It marked a fourth consecutive loss for City, leaving them five points behind leaders Liverpool.
Afterwards, a downbeat Guardiola said: "We have to try and win games again. Four in a row. We have to change things quick.
"The schedule becomes tough but it is going to happen when the players come back."
He added: "Maybe after seven years winning six Premier Leagues, maybe one year another team deserve it."
Speaking to BBC Radio 5 Live, he said: "This is sport. Sport is not always sunrises. It is not always good moments.
"Today in the press conference I was asked if it was the end of the era. I know people want that. I smell it for many, many years.
"What we have done in these years, people have said it’s so difficult, but if somebody would like to beat us it is going to happen because in the next 50 years we're not going to win all the Premier Leagues. It's impossible."