Sir Alex was speaking exclusively to MUTV. To see more of this interview, tune in to Matchday Live at 18:30 GMT on Sky channel 418.
Brazilian defender Rafael will be handed the task of stopping Cristiano Ronaldo in the Bernabeu tonight, as Manchester United take on Real Madrid in the Champions League Last 16.
Media speculation has been rife around how Sir Alex intends to shackle the Portuguese forward, particularly after Phil Jones successfully man-marked Marouane Fellaini in last Sunday's 2-0 win over Everton. But the Reds boss, speaking exclusively to MUTV in Madrid, has revealed it will be right-back Rafael who'll be asked to keep tabs on the former United ace.
"The responsibility will be with Rafael," Sir Alex said. "Rafael's maturity this season has been fantastic. Obviously Robin van Persie has been sensational but, outside that, Rafa and Michael Carrick are having their best seasons ever.
"He's Brazilian so he doesn't fear a thing. I trust him and have faith in him tonight."
Sir Alex admitted Ronaldo, who spent six seasons at Old Trafford, has progressed even further since signing for Real Madrid, but is confident he knows enough about the 28-year-old to negate his prodigous talent.
"He's flourished since he's come to Madrid," the manager said. "You'd have to be stupid not to respect his goalscoring record.