Ronaldo, 29, spent five years at Old Trafford before making his then-world-record transfer to Real in 2009.
The FIFA Ballon d'Or holder has since scored an incredible 177 La Liga goals in five seasons, and broke the record for the number of UEFA Champions League goals in a single campaign in leading the Spanish giants to success in Europe's premier club competition last term.
Despite his phenomenal record at the Bernabeu, Ronaldo has regularly been linked with a move back to United.
And - ahead of Real's International Champions Cup meeting with United in Michigan on Saturday, a game the Portugal captain will miss as he continues to recover ahead of the new season - he was coy on whether he could one day ply his trade in the Premier League again.
"Well, of course, 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," Ronaldo told MUTV.
"I loved it there, I was 18 years old, I won everything there.
"The people treat me like a god there, so I appreciate it and [in] the future, nobody knows.
"Of course I'm happy at Real Madrid - it's my home, it's my club, but Manchester treat me unbelievable so we never know."