The 17-year-old scored in the 80th minute to give the Blaugrana a hard-earned 1-0 win and close the gap over Real Madrid.
The 17-year-old needed less than a minute to break Athletic Club hearts and close the gap on Real Madrid atop the Liga table.
After 79 painful minutes from a cast of experienced players, Barcelona needed youth to salvage what could be a crucial La Liga win. Marc Guiu, making his club debut and second matchday squad appearance, bagged a late goal to give the Blaugrana a 1-0 victory after an otherwise frustrating contest. Joao Felix should have put Barcelona ahead after 10 minutes, but clipped Ferran Torres' squad ball over the bar — despite having ample space. Fermin Lopez, too, came close, only to see an effort well denied Unai Simon.
Marc-Andre ter Stegen had his moments in the Barca goal — notably diving to stop Nico Williams in the closing stages of the first half.The Blaugrana had further chances in the second half. Simon denied Felix twice within the first 15 minutes. Torres had a shot blocked by a lunging defender. Fermin Lopez skied one from 10 yards out. Lamine Yamal dragged one outside the post shortly after coming on.
And it was a debutant who finally took one. Guiu had been on the pitch for less than a minute when he burst in behind the Athletic defence, corralled a pass, and fired past Simon to put Barca just one point behind Real Madrid ahead of next week's Clasico.