In the second week of La Liga Barcelona will visit Real Sociedad, and statistically Real Sociedad is better than Rayo.
I think Real Sociedad will play more openly than defend against Barcelona. From head to head statistics Barcelona always win against Real Sociedad either at home or away.
I also feel confident, in the second match Barcelona will get a perfect win.
However, bookmakers estimate the probability of Barcelona winning as 2.1, which is less than 50% (approximately 45%). Therefore, if you are sure that Barcelona will win, then this outcome will be a value bet for you. I won't bet on this match (maybe I'll do it later live), but Real Sociedad or draw with odds of 1.67 looks more interesting.