Lamine Yamal made his Barcelona debut at 15 and has inherited the number 10 shirt from Messi.
He helped Spain win Euro 2024 and, now aged 19, could become the youngest player to win the European Championships and the World Cup.
"He's one of the best in the world right now," said 39-year-old Messi.
"I wish him luck because his success will be Barcelona's success.
"We'll try to keep him from playing at his best. Spain has a great team, not just him.
"He's a tremendous player - a global star. He's 19 years old and has his whole career ahead of him. I wish him the best, but we'll give it our all to ensure he doesn't become champion this time."
Messi attended a fans' event on Friday with Argentina goalkeeper Emiliano Martinez and Spain captain Rodri.
After the news conference, Argentina head coach Lionel Scaloni hugged Spain counterpart Luis de la Fuente, who was one of the instructors when Scaloni gained his Uefa Pro coaching licence in Spain.
Former England defender Rio Ferdinand and actor Kevin Hart hosted the event, while tennis legend Novak Djokovic and NBA star Kevin Durant joined Brady in asking questions.
Brady continued his ongoing feud with Logan Paul as they argued on stage before the record seven-time Super Bowl winner slapped the YouTuber turned WWE star, with New York Knicks player Karl-Anthony Towns splitting them up.
Image gallerySkip image galleryImage source, Getty Images
Image caption,After the news conference, Fanatics founder Michael Rubin and rapper Travis Scott also came on stage for a group selfie
