Ruddy returns to Goodison Park with Norwich on Saturday aiming to show what Everton missed out on.
The 26-year-old only played 80 minutes of Premier League action for Everton and was loaned out nine times during his five years on Merseyside.
Moyes has been impressed by Ruddy's development since leaving Everton with the goalkeeper breaking into the England squad.
"I'm a little surprised at how he has developed. But it was difficult with John," said Moyes.
"It was always going to be difficult for John to get games in front of Tim Howard.
"But he's done great. You rarely put young keepers in the team and when he went to Norwich it looked like a chance for him. He hasn't half take it.
"He's progressed under Paul Lambert and now Chris Hughton. He's in and around the England squad, so good luck to him."