Rayo vallecano has a very good defense, the only thing they lack is consistency but asides from that they are a very good team. I also thought it was going to be an easy win for Barcelona but to my surprise they ended up drawing the game. Like you said, these are things that happens to remind use that sometimes the outcomes are based on luck occasionally. I don't think we should expect much from Barca this season
Source: Google
Match results aren't just about luck; the experiences of previous matches between the two clubs serve as lessons for other clubs in understanding their opponents' strategies.
What Barcelona and Rayo Vallecano achieved wasn't entirely about luck; it's something that's happened before.
A draw isn't a bad result, and it's too early in the season to say there's no hope for Barcelona.