Manchester United manager Sir Alex Ferguson believes it is near impossible to bring Cristiano Ronaldo back to Old Trafford, as the Real Madrid superstar prepares to face his old club in the last 16 of the Champions League.

Ronaldo created waves when he said he was "sad" at Real Madrid at the beginning of the season, which immediately led to the forward being linked with a move back to his former club.

The fact that the 28-year-old still considers Ferguson a big part of his life, further fuelled those rumors. However, Ferguson, despite admitting he would love to re-sign his former protégé, admitted it would not be financially possible.

"Everyone goes on about that but I don't see it," Ferguson said. "He's got two-and-a-half years left on his contract and to try and buy that at the moment, you'd be talking about hundreds of millions.

"It rules out everyone, including Barcelona. I think Madrid are trying hard to get him to a new contract and I think that's happening.

"The glare of publicity he gets in Madrid is different to here. The players here, I think, are quite well protected -- especially in the training sessions.

"There are cameras everywhere at Madrid as everyone knows. I think he will miss a lot of things here (at United) but that's not stopped him progressing as a footballer. His progress has been brilliant."

Ferguson revealed he had no doubts over the fact that Ronaldo would one day be one of the greatest players in the world.

"He's a fantastic player and I knew he would do well and improve tremendously in Madrid," Ferguson said. "He was still youngish when he left us and it was obvious he was going to be a top player in the world, no doubt about that.

"When he first came, he was a boy who wanted to display all his tricks and technical abilities but, bit by bit, he overcame all the little flaws in his game because he's an absolutely rounded player.

"His last game of his last season for us... well in almost every one, he created five chances, sometimes more, by himself. He was unbelievable."