If someone signed Herrera from porto it will be based on potential.
He wasnt great for Porto (his first year) last season, but hardly anyone was. He also had inconsistent playing time, adapting to a new league.. one would expect him to be better and have grown from the world cup.
I saw him live all games in the world cup.. honestly, he was almost the MOTM everytime and we are talking games vs decent midfielders.. i mean he ran LAPS aruond the dutch and croatian midfield. It was incredible seeing his pace in real life; just like rakitic was amazing to me (even though i hated seeing him out of position) live.
Good potential, i think he can develop into a very decent player.. not many midfielders out there with his work rate.. its all about consistency for him now, the talent and technique is already there.