The Portuguese, who has two years to run on his current deal at Bernabeu, returned to Old Trafford for the first time last week as Real Madrid knocked Sir Alex Ferguson's side out of the Champions League.
Ronaldo has since been linked with a switch back to the Premier League leaders, and Sir Bobby believes the forward's talent has exceeded the norm and would like nothing more than the 28-year-old back at the club he "loves".
"Any great player would be welcomed back but Cristiano went beyond being just a great player," Sir Bobby told reporters at 2013 Laureus World Sports Awards in Rio de Janeiro on Monday.
“He scores phenomenal goals, loves the club and was just the right sort of player at Manchester United. I’m a fan of him like anyone else and on a personal level I would like to see him back. I can’t say anything more than that."
Ronaldo scored the winner for los Blancos last Tuesday, but refused to celebrate as he helped knock his former side out of the competition.
And Sir Bobby revealed how badly the Champions League exit had affected Sir Alex, and underlined the Scot’s determination to win the trophy again.
He continued: "Going out has hurt him a lot. You have to work with him daily to see that. If there is an opportunity and you miss it.
"But the desire is still in him to win that trophy again and I don’t see why he can’t. We have a great football club and Sir Alex is the greatest manager there has ever been."