It means he has scored his first league goal for Barcelona quicker than any other player in the club’s history.
Victory took third-placed Barca back within a point of leaders Real Madrid, who they face next Saturday.
Barcelona manager Xavi said: “I told him he would have one good chance. I love seeing the face of these kids. They have no fear.”
Earlier on Sunday, Girona won 5-2 at home to Almeria to go level with Real, who drew 1-1 at Sevilla on Saturday.
Guiu was added to Barca’s senior squad as coach Xavi has several players out through injury, including Robert Lewandowski and Frenkie de Jong.
A Joao Felix shot clipped the top of the crossbar from a Ferran Torres cut-back in the first half while Nico Williams had a shot saved by home goalkeeper Marc-Andre ter Stegen.
Just before the hour mark, visiting keeper Unai Simon saved Felix’s low striker before denying Lopez from the rebound.
Then 16-year-old winger Lamine Yamal, who became the youngest scorer in La Liga history earlier this month, dragged a shot wide before fellow substitute Guiu slotted the ball past Simon after being played in by Felix.
Guiu joined Barcelona’s youth academy, more commonly known as La Masia, in 2013.
With Argentina superstar Lionel Messi its most famous product, La Masia’s other graduates include current Barca coach Xavi, Andres Iniesta and Carles Puyol, along with Gerard Pique, Sergio Busquets and Jordi Alba.
Cesc Fabregas left La Masia at 16 to join Arsenal, while Manchester City boss Pep Guardiola is another notable member of La Masia alumni.
‘His parents were both in tears in the stands’
Spanish football expert Guillem Balague: Marc has been in La Masia for 10 years and is known as a striker who is more efficient in the box. He doesn’t link up greatly but he has something; he finishes very well.
He has only played seven minutes for the B team. He normally plays for the under-18s. Since he scored, he has gone from 40,000 followers on Instagram to 100,000 plus.
His parents were both in the stands and cried as soon as the goal went in.
This was quite a night for Barcelona’s youth – the team finished the game with two players who cannot go home with their own car, because they aren’t old enough for a driving licence.
And in addition, they also had Gavi, Fermin Lopez and Alejandro Balde, who have progressed through the youth set-up. Interestingly enough, all these youngsters belong to different generations; they didn’t play in the same youth teams.
When these players come through, it is often by accident. The accident this time was a lack of options. Barcelona’s transfer activity has been restricted by financial issues – they have only been able to sign Oriol Romeu, plus two players on loan, plus Ilkay Gundogan on a free transfer. And this weekend, they had six players out injured. This has opened the door for young players.
If he could, Xavi would have more mature players. But it is good news for Barcelona that young talent is replacing some of the more expensive stars of the past.
