I know this is old news to many but it's been swept under the carpet a bit by the media with the obvious exception of Martin Samuel who can see FFP and the constant attempts to preserve the old guard for what it is.
UEFA, since the formation of the CL, has seeded teams based on results in Europe by the clubs plus a small amount relating to how strong the league in which they played was. The period they use is the last 5 seasons. The current rankings are here;
https://kassiesa.home.xs4all.nl/bert/uefa/data/method5/trank2018.html
We are 8th while United are 11th. Not happy that teams were passing them in the rankings the old guard put pressure on UEFA to alter the method so that historical success back to 1955 (albeit weighted) was factored in. The method is here;
https://kassiesa.home.xs4all.nl/bert/uefa/calc.html
This means that the new rankings look like this;
https://kassiesa.home.xs4all.nl/bert/uefa/data/method5/trank10-2018.html
We are now 14th while United are 4th and these are the ones that will be used for seeding going forward.
Just another way a small number of teams, particularly Real Madrid, Bayern and United in my opinion are trying to secure their financial future whether they can compete in the current climate or not.