Well, in theory, the bookmakers had predicted that Barcelona would win, even though Rayo Vallecano is difficult to beat at home, we are talking about a notable difference between the two teams in terms of player strength and coach strategies, it seems to me that Barcelona is not in such good shape.
The championship has just begun, meaning there are some initial difficulties to overcome, and it won't be easy to understand what will happen from now on. However, I agree with the bookmakers who predicted this. Rayo Vallecano isn't a very easy team to beat at home.