Martinelli, 18, has burst onto the scene this season, scoring ten goals in 21 appearances, with his outstanding form drawing the attention of the Brazil national team.
Mari knows Brazilian football after a spell with Flamengo and the defender had been admiring Martinelli’s performances before his January move to the Emirates.
“Martinelli has shown he is a bit different to any other player in terms of his ability,” Mari told Arsenal’s official website.
“He has great vision and there’s real talent there in terms of his dribbling and his ability to choose the right play.
“His decision-making is improving all the time.
“Martinelli still has a lot more to show but he has incredible talent and I think gradually he’ll keep developing and become a top player.”
Mari added: “He has exploded on to the scene and is showing that he’s a great player with a lot of talent.
“Obviously he’s still very young, but he’s already had a taste of playing at the top level and he’s doing very well.
“I’ll say congratulations when I meet him, but now he just needs to keep it up and work hard because he’s really talented.”