The Reds were pitted against the Spanish giants in Thursday’s last 16 draw, with the first leg taking place at the Bernabeu on 13 February.
And Sir Alex says Ronaldo will get a warm reception when he returns to Old Trafford for the second leg three weeks later – even if he’d prefer not to be facing the Portuguese star.
The Reds boss joked: “I think the contract we had stated Ronaldo can't play against us! I think it's a disgrace if he plays against us – no loyalty!
“One thing for sure is he'll get a great reception when he comes to Old Trafford – and quite rightly. After that, they can boo as much as they like!”
Sir Alex has no hard feelings about Ronaldo’s decision to end a glorious six-year spell at United when he left for Real in 2009.
“Ronaldo came here as a 17-year-old and developed as a great footballer,” he said. “We're proud of the part we played in his career.
“He always had a hankering to go to Real at some point in his career and we helped him. I don't think it's disloyalty as I got another year out of him when he wanted to go.
“He honoured that and did fantastically that year, so he went with our blessing, really. My way of looking at it is we were lucky to have him for six years and the fans will share that.”