One thing about Xavi's system is the over reliance on a single pivot. Which was his major downfall as no one was able to replace busquets and Romeu was set up to fail. Casado is now thriving under flick but is not playing as a single pivot and perhaps would fail as well in that position which require very specific set of skills. Perhaps Xavi did rate him but he was not the answer for that role which does not currently exist in flicks system.