Everybody is assuming Pep will play the exact same system he has before (which probably wasn't exactly the same tbh). He may have assessed the Premier League and decided something different is required.
Joe Hart has proved time and again that he is one of the very best 'keepers in the world. Buffon rates him in the top 3 and I'd be inclined to agree with him. De Gea is very good, and I'd be glad to see Utd weakened by him leaving, but he is essentially a very good shot stopper, as are most Spanish 'keepers who don't deal with multiple ways of bombarding the box as in other leagues. Everybody went crazy over Courtois before he'd even played a game in England and one season of turmoil at Stamford Bridge has shown him to look less unbeatable than before. Joe has been praised over and over by the very best players in the CL who were unable to find a way past him. He is quick off his line, great in 1 on 1 situations and one of the most athletic shot stoppers you'll ever see. Even behind last season's supposedly dodgy defence he still kept 15 clean sheets. The only time he looks uncertain is claiming aerial balls when playing for England and probably because that back four is appalling defensively.
It makes me laugh (and cry) when I hear people suggesting he's not even England's best 'keeper. There are one or two others around the world I'd be OK having in his place, but none I'd actually consider swapping him for.