"I think stability at the back is always a great start for your team," Fergie said after the match.
"There was a lot of good concentration at the back, the team defended very, very well and I think that was key to it really.
"If we can get back to that foundation of not giving goals away, then we can do well."
On Ronaldo's goal, the United boss added: "It was unbelievable, a magnificent hit, absolutely fantastic. The shot took them all by surprise, coming from 30 yards or whatever it was. The goalkeeper had no chance really.
"Ronaldo needs to keep the referees on his side and not turn them against him. He's expecting a foul to be given every time.
"He's the best player in the world, he's going to be fouled and sometimes the referee's going to err on the other side. He's just got to accept that. If he does that, then he's a great player.
United will now play Arsenal in the last four, on which Fergie commented: "An all-British semi-final adds a certain spice to the tie.
"It's two good football teams - the game at the Emirates earlier in the season was a fantastic representation of the way football should be played.
"It should be a terrific semi-final and we're looking forward to it."