This is always a bit of a chicken-egg thing.
Pogba looks shit at United, if he were here, under Pep he'd be one of the best players in the league. Sanchez would be the same. They'd have clear instructions catered to their individual strengths, they'd have been drilled to know exactly where to be relative to the ball and in a structure where they'd have forward passing options and support nearby.
The reason they are playing poorly is not because they are bad players, their careers prior to arriving at the swamp put that notion to bed, but because they are managed badly.
Dani Alves sounds like he's being a right twat at PSG now they're out of the CL, but that's because he knows he's untouchable because he's Neymar's friend, and Neymar needs to stay happy in Paris. I'm sure under Pep he'd have probably fallen in line because he rarely or never spoke out of turn under him at Barcelona.
Rooney is one I would agree on, and Kaka seemed to be damaged goods at the time, the knee injury which ended him was already there in the summer of 2009.