As stated in the League of Legends European Championship rules, "Champions that have only been available on live servers for one week will be automatically banned." On the other hand, a new champion cannot be made available if a Competitive Week has already started.
These provisions — which are nothing new since Sett had suffered the same fate — aim to ensure that professional players have the time necessary to learn to play the champion and find compositions in which he excels.
In this regard, Lillia could not be played by the pros upon its release. Introduced to League of Legends with Patch 10.15, the Bashful Bloom took her first steps into the Rift midway through LEC Week 6.
Therefore, and by adding the two rules mentioned above, Lillia will only be made available in LEC from Week 8.
Riot has not yet clarified how these two rules will apply to the rest of the regional leagues or ERLs, but it is easy to speculate that the same provision will apply to them.
Moreover, as mentioned by Rioter and LEC Commissioner Maximilian Peter Schmidt in another Tweet, Yone will unfortunately not be made available during the LEC Playoffs.
We who were hoping to see Yasuo / Yone duos wave their blades on the botlane, we will have to wait a little bit longer...
While Yone has yet to set foot in the Summoner's Rift, a recent leak would seem to indicate the identity of the next champion to join League of Legends. She would be called Samira and could well be an ADC.