Barcelona managed to end the first half with a 0-1 lead after Lamine Yamal converted a penalty, but the second half proved to be a difficult one. The players struggled to penetrate Rayo Vallecano's defense throughout the second half, preventing Barcelona from maintaining their lead. Several mistakes allowed Rayo Vallecano to come back and equalize, so things were a bit more complicated for Barcelona.
Barcelona struggled to withstand the rapid pressure from Rayo Vallecano's attacks from all sides. This is clearly a problem Hansi Flick must address before the end of the match. Barcelona seemed to struggle to outwit the opposing defense and tended to give the ball away easily, but there's still room for improvement.
From my observations, the substitute made by Flick killed Barcelona game strength. Barcelona was playing pretty good before bringing in Rashford and Fermin to replace Olmo and Raphina. This gave Rayo space to make use of Barcelona weakness to outplay Barca and pressured their defense leading to the equalizer. Rayo lost quite some good chances to double their goal. The game is over.