The 29-year-old joined Real Madrid from the Red Devils in a €94 million deal in 2009 after spending six seasons at Old Trafford.
Ronaldo, who will not face United in Saturday’s pre-season friendly in Michigan due to fitness concerns, admits he sees his former club as a "second family" but reiterates that he is happy to remain at the Santiago Bernabeu for now.
"Manchester United are one of my favourite clubs, everyone knows that every time when I speak about Real Madrid I speak about Manchester United, I loved it there," Ronaldo told reporters.
"I was 18 years old when I joined, I won everything there, the people treat me like a god there so I appreciate that and, in the future, who knows.
"I remember when we played there against Manchester United in the Champions League. Their affection killed me, I was so happy.
"I had fantastic moments and I appreciate this club. For me, it’s one of the best clubs in the world and I have passion for this club.
"Some players I am still in contact with because, when I was there, I created a good relationship with everybody - not just the players, but all of the staff.
"It was like my second family there, so I’ll not forget that the people treat me good. I love Manchester United and to go back... With football you never know.
"Of course, I’m happy at Real Madrid - it’s my home, my club - but Manchester United treated me unbelievable so you never know."
Ronaldo also believes United will prosper under Louis van Gaal’s guidance following their worst-ever Premier League campaign last season under fired coach David Moyes, who had replaced Sir Alex Ferguson in 2013.
"It will be a better season for sure. My feeling is Van Gaal is a fantastic coach, he showed that all the time for Netherlands, the team played unbelievable at the World Cup, and I hope that Man Utd come back like the people know," he added.