While many United fans still sing Ronaldo's name, others have not forgotten he never hid his desire to swap Manchester for Madrid.
Ferguson persuaded him to stay for another 12 months and believes supporters should just be grateful that they saw one of the greats play at Old Trafford for six years.
"It wasn't disloyalty because I got another year out of him when he wanted to go the previous year," said Ferguson. "He honoured that and was fantastic for us. He went with our blessing.
"I was sorry when he left, don't get me wrong, but he always had a hankering to go to Madrid in his career and we helped him on that way.
"My way of looking at it is we were lucky to have him for six years and the United fans would share that.
"One thing is for sure, he'll get a great reception when he comes to Old Trafford and quite rightly so when he comes on the pitch - after that, they can boo all they like!
"It will be a fantastic tie. We've not had a really good European game for quite a while, having not qualified last year,' Ferguson added. 'This is a big opportunity for United to kick on and step forward in European football again."