Many inside the Spanish game have taken aim at the teenager's antics - especially off the pitch.
But Mendes, speaking with Mundo Deportivo, declared: "I don't understand all this fuss about Lamine Yamal.
"At 18, we were all young once. As president (Joan) Laporta said, we must support him and help him, because he is a valuable asset for the club.
"Lamine is the player everyone is talking about: he is unanimously recognised as a great talent, both for the present and the future."
On Yamal's approach to his profession, Mendes insisted: "He knows exactly what he has to do, both on and off the pitch. He works quietly, says little, and learns every day. He's a very intelligent and focused boy. Remaining humble and working hard is crucial to his growth.
"His greatest strength is letting his game do the talking. Every match delivers a masterpiece, as we saw against Club Brugge. Only players destined for the top level can do so with such ease."
Asked about comparisons with Lionel Messi and Cristiano Ronaldo, Mendes insists such debates shouldn't be made.
He added: "Lamine is unique and shouldn't be compared to anyone. But it's inevitable to look to role models like Messi, Cristiano Ronaldo, or (Andres) Iniesta, examples of professionalism and absolute dedication.
"If he continues on this path, he could become the best player of the next two decades, not just for four or five years."