Unai Lopez had Rayo ahead before Pedri and Dani Olmo struck for Barca after halftime.
Perez said: "The calendar is what it is. We believe it is excessive, nothing is going to change no matter how much we protest. It is no excuse, we have fallen physically and football-wise for the first time. With their level it is difficult to sustain it for so long."
"Barcelona's victory is fair, it annoys me to say it because it makes me angry. We have to regularly maintain what we did in the first half.
"There are two very different halves, in the first everything planned goes well.
"There are good things, surely if we had drawn, it would have been unfair. My feeling would have been that the first half is a mirror that we must look at ourselves throughout the season on any field and against any team."