There is a misconception about ranked play. It is the default mode for League of Legends, the most standard way to play. Normal is "practice" and nothing you do there qualifies you to play in ranked. Only playing in ranked, qualifies you to play ranked.
As such, rank is divided into many divisions of play and depending on your skill as a player (not with a champion, but as a player) you are placed into your appropriate division. As such if your an Iron IV player for example, its assumed you are a general nooby and are playing at the lowest skill level, again, as a player, not as a player playing a specific champion.
Every player before entering ranked must achieve 30 levels on their account in normal games, intended to assure you understand the rules and flow of the game, it does not assume that you are a professional player or that you are an expert with every champion, this is a significant amount of experience with the game, again, as a player, not as a player with a specific champion.
Every aspect of this game is geared towards the player and while there are mastery systems and some system bias towards playing champions you are familiar with, there are also automations in place that could conceivably force you to play a champion you are not familiar with. For example your mains get banned, you get autofilled into a position you don't normally play, or as a result of someone on the opposite team picking the champs you normally play. This happens to everyone from time to time and we all find ourselves forced to play positions and champions that we don't know anything about. The assumption is, that as a player with 30 levels of experience you should be able to pick up any champion or any position and play with a reasonable amount of skill and that is a part of the intended challenge for players.
For example I normally play Malphite Top or Nami as support. Now if I get autofilled as a Jungler, I will definitely be playing at a major disadvantage as I neither know the position well or the champions for those positions. Still, I'm a reasonably experienced player and its expected as part of the competitive scene that I will work it out.
