Of course even though it's a pre-season game this result is a warning to Madrid. Last season they also beat Madrid in the second match. This means that Barcelona is no longer like the beginning of last season. Xavi has indeed changed Barca for the better. They were able to prove last season ended in second position. In addition, this season's transfer market is used by Xavi to make Barca even more powerful. I think tomorrow they are candidates for the La Liga title.
Actually the threat will always be there, even though Barcelona struggled like last season. The key is they don't let their guard down and take lightly the opponents they will face, whatever club it is. The difference now is that the pressure will be even greater on Real Madrid. But if you look at the project for the long term, Real Madrid has an advantage over its rival, Barcelona. This match they also gave a little glimpse of what their line-up would look like next season.
Barcelona is Real Madrid's toughest opponent in La Liga, and it is Barcelona that will pose the greatest challenge for them. And Barcelona has made some big changes in the team. I think Barcelona has realized that there is no point in having competent defenders in the team if they cannot score enough goals. And Real Madrid should also realize that Barcelona is adapting to the approach that Real Madrid took in the last season.
So I think Real Madrid already knows very well what Barcelona is planning to do. So if anything, Real Madrid should be ready to face whatever Barcelona throws at them. But I also agree that they should not let their guard down. They should take Barcelona as a very serious opponent and also as a potential contender for the La Liga title.