"Certainly a part of his improvement is mental because he's more mature," he told the club's official website.
"His decision-making is cleaner, sharper, quicker. He was always a very talented boy but because he's intelligent he learns quickly from experience.
"I'm happy that I always gave him the confidence because he's developing very well into a very strong goalkeeper and today certainly nobody would question that he's one of the top five goalkeepers in the Premier League."
Arsenal return to action on Friday night when they host Coventry City in the fourth round of the FA Cup.