Based on what exactly?
In Pep Confidential, Pep and his team talk extensively about how, if a player is willing to work to improve, Pep will put in just as much effort as the player, and that one of his fundamental beliefs is that a players ability, down to the very basic techniques is not fixed, and can be improved on.
They talk about it in depth, and how Pep worked incredibly hard to turn Eric Abidal, who at the time was a player known only for his pace and physicality as a defender, who was on the brink of being sold, into one of the best left backs in the world, with great ability on the ball.
They stress that as long as the player is determined to improve, Guardiola will be committed to improving them.
And that's before even mentioning the fact that Mangala is not some league 2 player who ended up in our squad. There was a reason the club was prepared to pay so much money for him and why he was one of the highest rated young defenders around at the time.
He had a shit time under Pellegrini - like Nastasic, Lescott, Otamendi and actually Kompany too. If he's still here by September I'm sure he'll be twice the player he was last season, and Pep will put a huge amount of work in with him.