Cristiano Ronaldo recently said he still misses his mentor Sir Alex Ferguson, and if the Manchester United manager's words are anything to go by - the feeling is mutual.

Ronaldo made a sensational move to Real Madrid in 2009 for a world record fee, and has gone onto establish himself as one of the best players in the world.

Ronaldo, however, made his name at United, with the forward admitting he would have never left the club for any other team but Real.

With Ronaldo saying he is a little "sad" at Real recently, there were suggestions of United looking to buy back their former player, who is still revered at Old Trafford. However, Ferguson admitted the transfer fee required would make it next to impossible.

"I don't think it'll happen but I would like to think it would (be)," Ferguson said. "We'd be talking incredible amounts of money now. What do you think his value would be?

"He's never injured. He's definitely unbuyable.

"When does he ever miss a game? Who could afford him? One of the Russian clubs could buy him but do you think he'd go to Russia? He complained about the weather here so imagine him in the middle of winter in Russia!

"I speak to Cristiano quite regularly. He's always available to talk to, and he's always wishing us well. I think our result is the first one he looks for, no doubt about that.

Asked if he regrets not inserting a buyback clause when United sold Ronaldo to Real, Ferguson said with a laugh: "How do you know I haven't?

"He always wanted to play for Real, it was a boyhood dream, but I'm not saying he'll stay there forever. He might leave, you never know."

Ronaldo will travel to Manchester to take on Man City in the Champions League in midweek, and Ferguson had special words of praise for his former player. "He's an unbelievable player," Ferguson added. "One of the best, no doubt about that.

"We have had a great pantheon of players but no question Ronaldo compares with any of them. In his last season, he made five chances every match -- he's a brilliant player."