I don’t really understand your point here but sometimes a team might have a bad performance in their domestic league and still perform wonderfully well in the champions league take inter milan last season’s performance as an example, they didn’t perform too well in serie A but suprised the world in the champions league. Some teams may perform well in the league but fail to perform in the champions league just like barcelona did last season.
I think it was meant that if you let's say you lost points at the beginning of the season, then in such a fleeting tournament as the Champions League it will be difficult to then catch up by improving your game and results. As for a successful game in the national championship and an unsuccessful one in your own championship and vice versa, I would like to say the following, that this is very often a matter of priority, that is, if you missed the chances of a title in your own championship, then you can give priority to the Champions League games if everything is much more successful for you.