Goalkeeper Almunia says the 19 year old must now get tougher to become a true Premier League superstar.
He told The News Of The World: "Theo is ready to take on more responsibility. He has the support from the fans, the club and his team-mates.
"He is one of the best players in the squad. The only thing he has to improve is an aspect of character.
"He's a very lovely guy - maybe too lovely on the pitch.
"I keep saying to him he has to become more nasty. After this he will be one of the best players in the Premier League."