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
Balde is a good left back, and if not for the injury, he has the ability to become one of the best left backs in the world. He is good in attacking and defending, and he was one of the players who contributed to Barcelona's success last season. Balde was injured in training and will be out for four weeks, which is another problem for Barcelona because no other player will be able to do what he is doing. Gerard Martin can be managed, but he is not good enough to hold the position. If not Inigo Martinez last season, Barcelona would have lost many games when Balde was injured.
I am not a Barcelona fan, but I don't like it when players get injured because they are also human beings. Injuries have made some players look bad, which is why the Spanish coach isn't calling Balde during international competitions. Barcelona needs to get another left back to help Balde, if it's Gerard Martin, I don't think he can handle the pressure.