The Scot dismissed rumours linking the forward with a second spell at Old Trafford this summer and joked a machete or a machine gun is his plan to stop the Portuguese on Tuesday
The Portugal international has been heavily linked with a move back to his former club at the end of the season amid uncertainty over his future with the Spanish side.
The 28-year-old will make an emotional reappearance at Old Trafford this week in a Madrid shirt when the two clubs meet in the Champions League, but Sir Alex has described the prospect of the 28-year-old becoming a Manchester United player again as "false hope" and feels the forward is likely to sign a new deal with Madrid.
“There hasn’t been anything to encourage me to think Cristiano would come back here," the Scot told reporters.
"As far as I am aware, there is nothing in it at all. Ronaldo has got two years left on his contract and it seems to me it is all part of the negotiations that are going on about a new deal.
“Maybe his agent is playing the game. I think it is false hope. Cristiano will get a good reception. All our former players, especially the ones who have been great for this club, get a good reception when they return."
United host Real Madrid in the second leg of their Champions League last-16 tie on Tuesday evening, the first leg in Spain having ended in a 1-1 draw last month.
Sir Alex insists Ronaldo is at the pinnacle of his career heading into the fixture and joked of two ways that his side can prevent the forward from finding the net.
“He is a better player now than he was when he was with us because he has matured. He is 28 and at the peak of his career. From 28 to 32, with the fitness Ronaldo has got - he never misses a game and is never injured - he will be at his absolute best," he continued.
“I’ve got a plan to stop him: it’s called a machete. Plan B is a machine gun!”