There's a big difference in over-rated and not being particularly impressed by someone though. Sometimes you can only go on what you see. The few times I saw him for Juventus he looked bang average. Clearly he had some ability as you would simply never play for that sort of club if you didn't. But I don't think too many would have predicted he would make quite the impact that he has already.
The fact is that the 'professionals' at Juventus let him go on a loan with option to buy deal & the manager/coaches there clearly weren't all that fond of him too. So it's not just football fans who get shit wrong.
Obviously we know even less, but he's a completely transformed player here. Clearly he got stuck in a total rut at Juventus whereas playing with Son & Kane in this setup has revitalized him. A great move for the money & a lot of people (Including myself) were very, very wrong about him.