The Brazilian was once again in scintaliting form for the Gunners in Monday's win over Bournemouth in the FA Cup fourth-round.
And Keown thinks Martinelli can develop into a star in the same vein as City winger Raheem Sterling.
I just feel that this player is so mature for someone so young," Keown said on BT Sport.
"He's 18 years of age and we mustn't forget that. If you look at his vision. Everything he's linking really well, taking on board this new message of give-and-go. He's always on the move, he's a nightmare to pick up and Bournemouth just couldn't get near him.
The last player Arteta will have worked with in that position would have been Sterling and you saw the improvement of Sterling, so it's going to be a joy to watch this fella improve because of his starting position.
"This is where he is now rather than the bottom rung, 18-years of age."