Ferguson sold Ronaldo to Real Madrid for £80 million in 2009, and while the Portugal international has fulfilled his dream of playing for the Spanish giants, it is United rather than Real who have a better chance of winning the Champions League this season.
Ferguson was in Madrid on Wednesday to see Real's 2-0 defeat as Barcelona put one foot in the Champions League final, where they are likely to meet United at Wembley, following the Red Devils' 2-0 win over Schalke.
But even if United were to win the Champions League this season, Ferguson admits he would be "very difficult" to bring Ronaldo back to Old Trafford.
He said: "I hope [he regrets leaving United]. Ronaldo had a great time here, but he is at a great club now too. I spoke to him after the game [against Barcelona] and he is enjoying being in Madrid.
"He had a great time here, we got six years out of him, We got him as a kid and he went to Madrid a man and though he might not have been at his best he is still a great player.
"It's difficult to say whether he would come back, though. It would be very difficult."