"It was an unbelievable strike, fantastic, a magnificent hit," United manager Sir Alex Ferguson said. "They did well, defended well with good concentration. That was the key to it."
Porto had its chances but never looked to be capable of breaking down the United backline that welcomed back Rio Ferdinand after missing the 2-2 draw in the first leg through injury.
"Ferdinand's return to the side gave us a foundation which has been a great part of our success this season," Ferguson added. "In the last few weeks, we've been leaking goals and we had to get back to what got us to this point, to our form earlier in the season, not giving goals away."
United looked fresher than it did a week earlier when it was playing with two days' rest.
"That really makes a difference, particularly when you come into the tail end of the season," Ferguson said.
April 15 in Oporto
Porto 0 Manchester United 1. Goal: Cristiano Ronaldo 6.
Porto -- Helton, Sapunaru (Tomas Costa 80), Rolando, Bruno Alves, Aly Cissokho, Lucho Gonzalez (Mariano 31), Fernando, Raul Meireles, Lisandro Lopez, Hulk, Rodriguez (Farias 64).
Man. United -- van der Sar, O'Shea, Ferdinand, Vidic, Evra, Anderson (Scholes 78), Carrick, Giggs, Cristiano Ronaldo, Berbatov (Nani 68), Rooney.
(Manchester United wins 3-2 on aggregate)