As /u/MaxxBaer said if the referee sees the indecent in the game and makes a decision upon it then the FA will look into no further. This is because the FA don't want to retrospectively change referee's decisions and inadvertently show that the referee made a mistake.

This is why often there is controversy when a ref clearly sees the incedent but does not act upon it only to later claim in their match report that they saw nothing just so that the FA can make an action. As the ref clearly saw Luiz's challenge and even booked him for it the FA can't act any further without showing that the ref was in the wrong.