United are sitting out the elite competition for the first time since 1995 after a dismal last season in which they finished seventh in the Barclays Premier League and failed to qualify for Europe altogether.
Bayern are another of the Champions League's mainstays
Their recent record includes winning the trophy in 2013 and final appearances in 2010 and 2012
They were semi-finalists again last season.
Yet Guardiola is aware that can count for little if standards are not maintained in the Bundesliga.
Asked about United, Guardiola said: "It is a good lesson for the big clubs if we think we are unbeatable, that we are so strong.
"Every year we need to make the good things or maybe next season you could be out.
"That is why football is magnificent
Every single week you have to be ready.
"The local competition - the Premier League or the Bundesliga - is the most important because that shows you every week you are ready."
Guardiola, the former Barcelona boss, was speaking at a press conference at the Allianz Arena to preview his side's opening Champions League Group E game against Manchester City.
The German giants have been drawn in the same group as the Premier League winners in three of the past four seasons.
City, now with two Premier League titles to their name, are a developing force and Guardiola believes they are capable of winning the Champions League.
He said: "Manchester City won (here) with (Vincent) Kompany, (Yaya) Toure, (David) Silva, (Edin) Dzeko last year - fantastic players.
"They needed to win here to learn something, to learn they are able to win the Champions League.
"Last season they were able to win the Champions League and this year they are ready to win the Champions League, like eight, nine or 10 teams more who can win this trophy
We would like to be one of those teams.
"City were a strong team last year
That is why they won the Premier League and deserved to."
Source : PA
Source: PA