Why am I getting demoted after losing one game?

So I woke up today thinking lets play a ranked game whats the worst that can happen I just got promoted to Gold I last night so this can't go wrong. But today we are playing a game and unfortunately riot has not decided to touch Master Yi in any way and want to leave him how he is so I lost my first game at 0 LP and instantly got demoted. When I checked Op.gg I had Plat 5 mmr. So I'm just confused why I had no demotion protection. All I want is either Riot to fix their ranked system or just get rid of Master Yi because all it takes is playing him to win any game. But unfortunately Riot has come to a point where all they want is Money and wont change a champion unless they are played in pro play or they plan on releasing a new skin for them.