I have seen this so many times. It's absolutely no way to justify his transfer.
Pep's track record on defenders is awful from Barca to Bayern to City. Would you buy a car because just because someone else wanted it, even if they already bought a load of cars that were already on the scrap heap?
Maguire is a good defender, I've always thought that. Even when he was relegated at Hull, you could tell that both he and Robertson were very good and much better than the team they were playing in.
But he is just not good enough to be the linchpin in the backline of a team that wants to win major trophies. This just is like Di Maria, Veron, Lukaku, Forlan or even Pogba, good players that for whatever reason don't suit the club. Look how Bonucci faired when he went to Milan, it wasn't working, he went back to Juve a year later, and he's one of the best defenders of the last 10 years.
It happens, if he doesn't start performing to the standard you'd expect from an 80m centre back then you eventually have to cut your losses.