Pep inherited a broken team, that had finished the La Liga 18 points away from Real Madrid. He sacked the 2 best players of the squad, Deco & Ronaldinho, and didn't sign that many players that summer, except Dani Alves who had been signed way before he turned to be the coach.
And with that squad, no one in Spain thought he would even win the league title, as Real was seen as a much better team then. He won the treble. He turned Messi into the player he is now. And he turned a player like Busquets, who wasn't even considered a real promise of the youth team on the 3rd division, into a starter and one of the best DMFs in the world.
As a Barcelona fan, I can guarantee you that people at Barcelona didn't trust at all in a midfield with Xavi and Iniesta at the same time. In fact, they had never been solid regular starters at midfield for years. People expected to have a midfield with a blend of muscle and stamina with skill, that's why Deco was the starter and Iniesta came from the bench up to then.
Guardiola completely changed this mentality and the players in the squad and turned all of them into world stars and probably one of the best teams ever. If people don't give credit to this, I don't know who will be credited and for what.
Compare Pep to Sacchi, who is regarded as one of the best coaches ever. Sacchi only won silverware with Milan, and much less silverware and during a lot less time than Guardiola. He also had in his hands one of the best squads ever, with probably the best 3 european players at the time (Van Basten, Rijkaard, Gullit). Pep has delivered a lot more than Sacchi in terms of results. There's the romantic idea that a manager who achieves something big with a smaller side is a better coach, but that's not true. Managing a big team is way more difficult due to a number of reasons: expectations are higher, fans are a lot more demanding, every loss counts more than 5 wins, etc... There's a reason why many of those coaches who achieve something with a lesser side won't succeed in a big team. Last year Leicester did a miracle. Rainieri made it possible, but even though we all absolutely love and respect him for that, do this make him a stellar coach, a genius? I don't think so. He's got my utmost respect and admiration but Rainieri is a good coach while Pep, to me, is a genius. With all the good and bad things that come with it. Pep has changed the mentality of a whole generation, like Sacchi did back in the 90s or Cruyff did during his whole life.
Just don't expect him to win everythig all the time.