The 29-year-old joined Real Madrid from United in an £80 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, admits he sees his former club as a “second family” but reiterates that he is happy to remain with Madrid.
“It was one of my favourite clubs, Manchester. Everyone knows that every time when I speak about Real Madrid I should 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 the future, nobody knows.
“I remember when we played there against Manchester in the Champions League, they kill me, my heart 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 the staff around.
“It was like my second family there, so I’ll not forget that the people treated me good. I love Manchester, and to be back, I never know, you never know. Football you never know.
“Of course, I’m happy at Real Madrid, it’s my home, it’s my club, but Manchester treated me unbelievably well so we never know.”
Ronaldo also believes United will prosper under Louis van Gaal’s guidance following their worst-ever Premier League campaign last season.
“It will be a better season for sure. My feeling is he [Van Gaal] is a fantastic coach, he showed them all the time for Holland, the team play unbelievable, good football and I hope that Manchester come back like the people know,” said Ronaldo.
“A strong United, playing nice football, with a lot of goals. I think Van Gaal is going to do a fantastic job, he’s an experienced manager and I think he’s going to do well for Manchester.”