Ronaldo takes on his former club for the first time since his world-record £80million move to Real Madrid, in the Champions League last-16 first leg in the Bernabeu.
Sir Bobby told the Daily Star Sunday: “I love Cristiano. I was really, really sorry when he left us but we couldn’t stop him leaving.
“Playing for Real Madrid was something he had to do in his football life.
“But I always wonder what sort of treatment he is getting at Madrid and whether he can match what happened at Old Trafford and the affection that was shown to him.
“They loved him at Old Trafford and still do. People are asking me, ‘Is there any chance, if Real Madrid fall out with him, that maybe we could bring him back?’
“I always say, well, we’ll have to wait and see. He is a great footballer – and we will enjoy playing against him.”