The 25-year-old World Player of the Year spent six seasons at Old Trafford, winning the Premier League three times as well as the Champions League.
"Of course I miss playing for Manchester United," Portuguese international Ronaldo said in an interview with Britain's News of the World newspaper.
"I played there for six years and that's a long time. I am still interested in watching Manchester United and, you never know, maybe in the future I could return to play there. It's always possible.
"I am really happy at Madrid, and everyone knows this is my club, but of course I miss Manchester United, the boss, the players because I left family there."
Ronaldo, who joined United as an 18-year-old for 12.24 million pounds from Sporting Lisbon in 2003, played 292 times for English club scoring 118 goals.
He has since made a bright start at Madrid, scoring 17 times in 17 domestic and European matches, but said he retained an interest in the Premier League.
"I think the English league sometimes is more exciting than La Liga. People love the game more in England. The fans love the game more.
"In my opinion, the Premier League and La Liga are the two best leagues in the world, but in England the players are stronger."
