I probably underrated Griezman when he was at Sociedad but I still don't rate him as most people do. To me he's a bit like Theo, with better technique minus the blistering place. I reckon he'll always be a tier below the real top class footballers.
Talents like Correa,Martial etc on the otherhand you just know are going to be among the very best footballers in the world baring any injuries.
BTW Santi Mina is probably worth keeping an eye on for Valencia, was really impressed by him yesterday.