The Real Madrid star enjoyed six successful years at Old Trafford, and the Scot says everyone at the club is in no doubt as to who is the best player in the world.
The Real Madrid forward is being tipped to win the Ballon d'Or after helping Portugal secure a place in the 2014 World Cup by scoring four goals over two legs against Sweden, including a hat trick away from home to continue an excellent start to the season for club and country.
Ronaldo, who left United for Madrid in 2009 in a world-record transfer, signed a new contract at the Bernabeu in September, keeping him at the club until 2018, but Moyes has not given up on coaching the 28-year-old.
"Everyone at Manchester United would tell you he is the best in the business anyway," Moyes told MUTV. "I have not been fortunate enough to work with him, but there is still hope some day it might be possible."
Meanwhile, the United boss has also warned the club's Premier League title rivals that the reigning champions are finding form.
"I think that everybody knew what it would be like," he said. "We knew it would be a change of manager and we knew there would be problems no matter what, whether it be winning games, new personnel, but overall the players have been great.
"They have shown me what they can do. I’ve said many times Manchester United have shown over the years they have been pretty slow starters, my history at Everton I was a slow starter, and hopefully we are beginning to find our feet and a bit of rhythm."