"It was a very important moment in my career because I realised a dream that I had had since I was very young," Lionel Messi told Barca TV in an interview.
"I had fought hard to reach this moment, and it was very special."
Barca's Dutch coach Frank Rijkaard brought on Lionel Messi for Fernando Navarro in the 75th minute of the inaugural match at the Dragao stadium in 2003, against a Porto side managed by Jose Mourinho.
Wearing the number 14 shirt, the floppy-haired youngster showed a few flashes of his talent, but Barca lost the game 2-0.
"We enjoyed the game, the experience with the first team, travelling and so on, because it was new for all of us, but when we returned we kept our heads focused on the Juvenil A team," Lionel Messi said of the trip made with some of his youth-team colleagues.
"(My parents and the coaches) told me to enjoy everything I was experiencing and that there was no reason to change, that I should continue in the same vein.
"I think I am still the same person. I still view things the same way, but many years have passed now and I have continued growing, learning and valuing everything."
Now 26, Lionel Messi has won four consecutive World Player of the Year awards, three Champions League winners' medals and six La Liga titles and has smashed various scoring records in the process, becoming Barca's leading scorer of all time in the league.
He is in the running for a fifth World Player of the Year award in January but has been knocked off his stride by a hamstring injury that is expected to keep him on the sidelines for six to eight weeks.
Lionel Messi is working on the first part of his recovery in Barcelona and will return to Buenos Aires for the latter stages. He is not expected to be back in action until after La Liga's two-week winter break at the end of December.
Barca top the standings, three points ahead of Atletico Madrid, and are six ahead of arch-rivals Real Madrid in third.
They have already qualified for the last 16 of the Champions League which resumes in February, and meet third-tier Cartagena in the last 32 of the King's Cup in December.