However, the Real Madrid and Portugal star also said he was happy to stay in Spain for the foreseeable future, suggesting United might have to wait before re-signing the player they sold to Real for a world record fee of £80 million back in 2009.
"I miss English football," Ronaldo said during a promotional trip to Singapore.
"For me, it was one of the best years in my career when I was there in Manchester United. Everyone knows that it is a club that is still in my heart and I really, really miss.
"But now my life is in Spain. I am enjoying playing there too.
"Part of my life is there, but in the future we never know. I am really, really happy in the Spanish league."
Real's failure to win major silverware last season and reports of division in the dressing room have prompted speculation Ronaldo, whose contract runs until June 2015, may be seeking a move away from the Spanish capital.
Club president Florentino Perez has described Ronaldo as "priceless" and said he would stay and help Real win the 10th European crown they have been chasing since their last continental success in 2002.