The 19-year-old was a revelation this season after joining from Brazilian fourth-tier side Ituano, having scored 10 goals in 26 first-team appearances.
Now tasked with being a mentor for his compatriot, Willian thinks Martinelli is almost ready to play for Brazil.
"It's because of his quality and also without the ball you can see that he is always trying to help his team and do the job without the ball. So I think he can be one of the best players.
"I think when he starts to play again, I know he is injured now, but when he starts again, it's about time for him to go to the Brazilian national team."