Arteta says friends had raved about the ability of Brazilian teenage prodigy Martinelli even when he was No 2 at Manchester City, but thought there was no way the 18-year-old would be that good.
He said: “I heard a lot of things about Gabby before I joined and the moment I saw him training it confirmed all the things I was told.
“I was told, 'You will see this kid, how he goes for every ball, his fighting spirit and the quality that every time he is in front of goal he's a threat'.
“I was told I would be surprised how prepared he is to compete with every player in the squad.
“I just thought, 'Yes, but he's 18'. But when you see him in big games, how he goes about everything, it's surprising.
“The way he competes, how brave he is with every decision he makes on the field is very unusual for an 18-year-old."
Martinelli added: “The risk is expecting too much too soon — but not just for the fans.
“We cannot give him the licence to think he has to put himself under that pressure. He needs to play with the freedom that he has."