Every team against Barcelona tries to show their best performance, and they also try to defend very well because they know that Barcelona is an attacking team and will create a lot of chances. Rayo Vallecano were lucky to play well in defense and, moreover, they were lucky to score a goal. Yes, you are right that sometimes such matches happen, and Barcelona expected to get an easy win here, but afterwards they couldn’t pull themselves together and score again. It’s disappointing to drop points at the very start of the league, but that happens too.
Unfortunately, the latest news is bad news for Barcelona, as their left-back, Alejandro Balde, suffered a hamstring injury during the training. He's likely to be out for three to four weeks. This will certainly present a challenge for Barcelona's defense, especially at left-back. They may include Gerard Martín as their main LB or another alternative defender to strengthen their position. 
Source: Alejandro Balde has suffered a 𝐢𝐧𝐣𝐮𝐫𝐲