The Portuguese winger's finish - likened to Lionel Messi's in the Champions League against Arsenal - came in a pressure situation, which only added to its audacity.
"I must say the finish was absolutely brilliant," Sir Alex told MUTV. "To have the audacity to try that at the stage of the game tells you about his courage.
"Not many players would have tried that, particularly from such a tight angle. It was fantastic - 10 out of 10 for doing it."
Ryan Giggs scored twice from the penalty spot - the second earned by Nani - and praised his younger team-mate for showing his "talent and temperament".
"With ten minutes to go and to be one-on-one with the goalkeeper, to chip it over him like that shows great confidence. But he’s the talent he has throughout the season. We were all pleased to see it go in, and he deserved it."