Martinelli arrived at the Emirates from Brazilian third-tier side Ituano last summer, and the attacker has already hit the 10-goal mark since making the switch to England.
The teenager has quickly become a fan favourite in North London, and Leno believes that Martinelli has the right attitude to become a future superstar.
Speaking to SoccerAM, Leno said: “He can become a world class player. World class 100% because he has talent, he has quality.
“But I think his biggest strength is his mentality. He is working unbelievably (hard). I remember in our bad time when we struggled a lot, he was working so hard, he was one of the leaders and at 18 years old he came from nowhere, nobody knew him before and to play like this, to behave like this, is very good to see.
“I think if he keeps going like this he will be an unbelievable player.”
Martinelli was recently listed among the top 10 of Football Talent Scout’s list of the top 50 teenagers in world football, alongside fellow Gunners Bukayo Saka and William Saliba.