The Portugal international faces his former club for the first time on Wednesday evening as Jose Mourinho's side host Sir Alex Ferguson's men at the Bernabeu in the last 16 of the Champions League.
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"To play for City not United? I’m not going to do that. Why? Because my heart’s in Manchester United, that’s why," Ronaldo told The Sun.
"I can say it would be very, very, very, very, very, very, very, very, very, very, very, very difficult to play at Manchester City. Very difficult because of Manchester United."
Ronaldo has reiterated his desire to remain with the Spanish champions beyond the end of the season but also hints that he would be open to a return to Old Trafford in the future.
"I am a player of Real Madrid and I’m happy here, my family is happy and everything is going well for me here. The future? I don’t know — only God knows the future," Ronaldo said.
"I can’t say I’m never going back to Man United — but I can say I’m good here and I want to be here.
"Let’s leave it at that and see what God gives me in the future — but I repeat I am happy here at Madrid."