Barca won 6-1 on Tuesday night, with Marcus Rashford also scoring twice in Montjuic.
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Afterwards, a beaming Fermin said: "Very happy. We needed a win like that. We finished on a good note, and now we're looking ahead to the Clásico.
"It was my first hat trick, it's a dream come true and I can keep working.
"Now I'll take the ball so my teammates can sign it. I'm very grateful to the entire team and for keeping up that momentum.
"I feel comfortable in that attacking midfield position. I can improve my game. But I'll be wherever the manager needs me."