There are many teams that perform well against strong teams and perform poorly against less strong teams. Sometimes strong teams do not know the past analysis of weak teams and their players in detail so during training, the match situation goes against them due to not knowing the strengths of the opposing team players. If they were aware of other strong teams and their players in advance and practices their strategies to counter them in advance it would be easier to play against them. The same situation was seen in the case of Barcelona, they performed poorly against Rayo Vallecano. But despite being a weak team played very well because the Vallecano players were aware of Barca strengths and went to the field after studying the players of the opposing team.
I doubt that Barcelona didn’t study Rayo Vallecano before this match, but that doesn’t change the fact that Rayo Vallecano have an excellent defense, and they should have expected that they would defend. But Barcelona didn’t try to force the game until the moment Rayo Vallecano equalized, and after that they were unlucky because they couldn’t adjust for the rest of the match. So in this game, the draw became possible only thanks to the hosts’ defense.
There is no excuse for Barcelona drawing to Rayo Vallecano, the home club were so good on the match day. They played so well and exceeded Barcelona’s expectations, defended so well and deserved to get atleast a point from the game. Probably Barcelona underestimated Rayo Vallecano going into the game and were stunned by how well they played.
I agree with you that there's no excuse for Barcelona 1-1 draw against Rayo Vallecano because Barcelona played with their full squad and Barcelona believed they would defeat Rayo Vallecano
at Rayo Vallecano home soil and take all three points
, but Rayo Vallecano surprises Barcelona with an amazing performance, which Barcelona managed to earn a point against Rayo Vallecano with the help of the referee.