Manchester United boss Alex Ferguson believes Cristiano Ronaldo could play for the club again one day, as he praised the Portugese star's commitment during his six-year spell at Old Trafford.
"He may come back, you never know," Ferguson told Britain's Press Association in Kuala Lumpur where the team are on a pre-season Asian tour.
"Sometimes it happens that way. He loved United. If you look back at the six years he had with us, he never missed training."
Ronaldo became the world's most expensive player when he moved to Real Madrid on a six-year deal worth around 94 million euros (131 million dollars) and he made a low-key debut against Shamrock Rovers in Ireland on Monday.
Still just 24, Ferguson was quoted as saying Ronaldo's best was yet to come, describing him as the top player in the world by a long shot.
"His best is yet to come. He's only 24. He came to the right club at the right part of his career," he said.
"He could have gone to Real Madrid, Barcelona or other clubs. But he came to the right one and appreciates that. It was always his intention to make his move.
"There's no doubt he's a huge loss and I think it's a challenge for us. It won't be the same without Ronaldo, it can't be.
"He's the best player in this world by absolutely miles, streets ahead of Lionel Messi, and Kaka," Ferguson added.
Madrid courted Ronaldo all through last summer but he decided to stay and try to help United defend their 2008 Champions League title, featuring in their final loss in May to Barcelona.
Although United retained their English Premier League title, Ronaldo has said he made up his mind to leave in May 2007.