Guiu was introduced with just over 10 minutes left at the Olympic Stadium, sliding home the winner 33 seconds later after being fed by João Félix to give Barca a narrow 1-0 victory.
“Marc is a kid from the academy and he made the difference,” Xavi said in a news conference. “I am happy for him and with the three points, which are really important.
“I thought of him in that moment because he is a goal scorer, he has a spark and I like him personally. I don’t have any problem looking at what we have in-house. The complete opposite. I want to give them the confidence that I was given at 17, 18 years old.”
Guiu is the latest youngster to emerge from Barcelona’s academy and make an impact on the first team.
“The thing that surprises me with this generation is that they have no fear,” Xavi added of Barça’s youngsters. “Maybe it was different in my day. They have that spark. They look at me with a face that says: ‘Put me on, I’ll show you that it will go well.’ When we put them on, they’re ready, they show character and bravery.
“I saw it in Gavi, Balde, Lamine, Fermín… and I saw it in Marc. There was no fear in his face.
“I told him he would get one chance. He gets one chance and he took it. I felt he was ready. I am really happy for Marc, he is a good kid who works hard.
“To score with his first touches is extraordinary. I’ve congratulated him just now and you can imagine how he is feeling. It’s quite possibly the best day of his life.”
Guiu, who joined the club’s academy in 2013 and is a Spain U17 international, enjoyed the moment.
“I can’t believe it,” he told reporters. “I am enjoying the moment. I didn’t imagine this. I have been working my whole life for it, taking advantage of every opportunity and… I won’t sleep tonight.
“I have dreamt of this many times, almost every night. I am so proud. It all happened really quickly, I hardly had chance to think about it. It was intuition.”
Guiu was handed his chance because of injury problems at Barcelona. Robert Lewandowski, Frenkie de Jong, Pedri, Jules Koundé, Raphinha and Sergi Roberto were all sidelined for the game against Athletic.
Xavi said neither Kounde nor Roberto will return for this week’s home games against Shakhtar Donetsk in the Champions League or Real Madrid in LaLiga, but it’s a case of “wait and see” with the other.
Ahead of Saturday’s Clásico, Barça are now just one point behind Real Madrid, who are level at the top of the league with Girona.