In my opinion, Barcelona did not show anything special in the game with Real - in the first half, I would even say that Real had more moments of domination, but the conversion of chances and successful counterattacks in this game turned out to be on the side of Barcelona. After 2-0 the game was done, but note that the second half of the game was technically tied 1-1.
Let's say that Barcelona will play just as well against United, but it may turn out that United will score first and the game will go according to a different scenario.
It seems true after I have watched the highlights again and again. Real Madrid was not performing so badly at that moment but counter attack makes real Madrid a bit surprised. No problem for real Madrid. Losing in a game was a common thing as long as you will not try to repeat the same mistake again in the future. Real may know how to prevent Barcelona in the future. There will be el classic to come next months. This gonna be a very important match for both caused by winning the match will make the club even closer to the title of la liga.
This defeat can no longer be corrected since it was the final, but everyone understands that the most important titles of the season are not yet played. I hope that this is the defeat that will benefit Real Madrid - it would be worse if Real realized that it needs to be more focused or something to change, for example, already in the 1/8 finals of the Champions League. Now there is a lot of time ahead and you can disassemble the mistakes made and become better.