Lamine Yamal, at the age of 16 years and 243 days old, was given the nod down the right for the Catalan outfit against Napoli, and he played the full 90 minutes of what proved to be a 3-1 home success for Xavi's side.
Cubarsi also played the full game in the middle of the defence, with the Spaniard aged 17 years and 50 days old.
Barcelona were level at 1-1 with Napoli ahead of the second leg, and they booked their spot in the quarter-finals of the competition courtesy of a 4-2 aggregate win.
The Catalan outfit became the first team in Champions League history to start two players aged 17 or under in a knockout fixture in the famous competition.
Fermin Lopez, Joao Cancelo and Robert Lewandowski were on the scoresheet for Barcelona, who have advanced to this stage of the competition for the first time since 2020.
Yamal has been a vital player for Xavi's side this season, making 38 appearances in all competitions, scoring six goals and registering seven assists in the process.
Cubarsi, meanwhile, has made 12 appearances for the first team in 2023-24, and he has started each of his side's last seven matches in the top flight of Spanish football.
«Cubarsi is a very rational kid, a humble guy, calm, down to earth. He has an incredible future, we're probably looking at a centre-back who will play at the maximum level for years to come,» Xavi told reporters in the aftermath of the clash with Napoli on Tuesday.
«My pulse does not go up when Cubarsi has the ball. That's the best summary for him.
»Cubarsi does not get nervous. He's very calm, his ball output is brutal, he
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