Because we have Busquets and Rakitic and a Iniesta who can't play over half of the games next season.
A player who can play with skills similar to Iniesta will be crucial no matter what the system is, imo. The suggestion is Coutinho - he can play as an 8, a 10 and as leftwing (+maybe rightwing). That would be an asset across most systems.
Let me rephrase my question. How do you know that Valverde's system needs an Iniesta role?