I don't see Barcelona winning the Champions League because the rating of their performance last season is very low. Even winning La Liga is luck to me because Real Madrid's attention was all on the Champions League and to some extent Madrid is also not serious, they played well during the first League against Manchester City and second leg i wonder what happened and they were clear beating by the city to qualify for the finals, I believe Barcelona should stick to La Liga since the Champions League is truly very competitive if you are not prepared, be prepared to dance to your own song outside of the league.
I believe we should consider more than just how they performed in the previous season when assessing how they will perform in the upcoming one. Barcelona's performance in the Champions League last season was not great, but I don't think it was so poor that they shouldn't be considered one of the favorites to win it the next season . Personally, I think Barcelona will give it their all in the Champions League the next season.
Barcelona won La Liga last season, but I don't think it was luck; I think it was their hard work because they snatched 10 points away from Real Madrid in second place, and once again, these aren't the only two clubs vying for the league. Therefore, I don't think it is enough to persuade me considering that Real Madrid's focus was on the Champions League. Teams' success in the Champions League does not imply that they neglected their domestic league if they perform poorly there.
It is specially important when you think about the situation at hand, because we are talking about a situation where teams play at the league, then play at their domestic cup, and then they also play at UCL as well. You can play at the league games with bench players if the team you are playing is not that great, and you can certainly do that at the cup games because it is not cared about all that much, but during the UCL games you need to play your best eleven since it is going to be very few games.
You play 6 games at group stages, 2 at 16, and 2 at quarter, 2 semi finals and 1 finals. That means a total of 13 games even for the winner, it is very important, and after group stages you need to be better on aggregate as well. This is why bench matters, you need to make sure you have what it takes.