United face the Real Madrid superstar on Wednesday, four years after flogging him for £80million.
Boss Fergie said: "If you were to buy him back, you are talking hundreds of millions.
"That rules everyone out, including Barcelona. I just can't see it happening."
Ronaldo, 28, who moved to the Bernabeu in 2009, has been heavily linked with a switch back to Old Trafford.
But boss Ferguson declared: "Everyone talks about that but I don't see it. Cristiano has 2½ years on his contract and I think Madrid are trying hard to get him a new contract."