First of all, he's 18 years old. Still in his teenager years. There is no teenager in the world, who can perform constantly on a high level like an adult professional athlete. It's completely normal to have a low phase, especially in his age.
In addition, he basically plays every third day without a pause thanks to the lack of alternatives. He's much overplayed, not only physically, but mentally too, playing without a break.
I have no doubt, that he'll have a worldclass career at barca, because he has the potential and abilities, despite current poor performances for said reasons.