It will be the Portugal international's first visit to his old club since his £80million move to the Spanish capital in 2009.
The two clubs' last-16 tie is delicately poised after a 1-1 first-leg draw in the Bernabeu, but Beckham has warned Ronaldo will be an even bigger threat than usual given the occasion.
"When you go back, you enjoy the occasion, of course, and I am sure it will inspire Cristiano," said the Paris Saint-Germain midfielder, who experienced returning to Old Trafford himself with AC Milan in March 2010.
"Many things inspire great players and him going back will be a special night for him. The fans love him and it will be a great occasion."
Beckham has played for both United and Real Madrid during his career and believes it is difficult to predict a winner.
He added: "United did very well at the Bernabeu and I am sure they will do very well at Old Trafford. It will be a great game to watch. It is hard to say who will win. Two great teams with great players."
Beckham made his second appearance for PSG in Wednesday night's 2-0 Coupe de France win over Marseille.
And the former England international is refusing to rule out extending his stay at the Parc des Princes beyond his current short-term deal, which is due to expire at the end of the season.
Asked if the contract ends in May, he said: "Not yet it doesn't."