The Catalan won 14 trophies in four seasons in charge, before moving on to Bayern Munich for three years to cement his status as a leading thinker in the game.
De Bruyne, who joined City from Wolfsburg in 2015, signed a six-year deal at The Etihad in January. Not many teams could lose a player of De Bruyne's quality and still be competitive.
City have hardly struggled without De Bruyne, but compared to last season's cruise to the title, a draw at newly-promoted Wolves and a tight 2-1 victory at home to struggling Newcastle have represented a challenging start to the campaign.
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He told Universo Valdano: "He is really good, a very dynamic player. I'm sorry, he's not for sale".
'The other day, I met his parents and you often understand how the children are when you know the parents.
For now, though, Guardiola insists he is very happy at City.
"When Jorge Valdano (interviewer) asked me about selling (De Bruyne) for £250 million, I say no, but it's my personal opinion".
The Manchester City playmaker is a key performer in Guardiola's team and stood out last season in the club's Premier League-winning campaign. "I'm well protected, I don't feel judged".