Ronaldo played at Old Trafford for six seasons before his world-record move to Spain.
As arguably the second-best footballer on the entire planet, much was expected of Ronaldo.
Real Madrid needed to score at least once, having conceded an away goal in the first leg. With United a terrific attacking force at home, they were always likely to get a goal themselves—which would again leave Real needing two to go through.
Ronaldo is Real Madrid's top scorer this season and is their biggest threat from set pieces, on the counter-attack and, often, in general open play.
So he was seen as the go-to man to get them through the tie.
It didn't quite happen for him during the game though, yet he somehow still came out as the match-winner.
In the first half he was well shackled by United right-back Rafael.
He limited Ronaldo's involvement in the opening 25 minutes or so to rare forays down the left wing where he was unable to trouble the centre-backs or goalkeeper with crosses or shots.
A four-man tactical switch on Real's part saw Ronaldo pop up more centrally in the latter stages of the first period and he instantly had more impact. He began stretching the defence and attracting Rafael to drift infield, leaving space for Fabio Coentrao to exploit down the flank.
The opening 10 minutes of the second half changed not only the dynamics of the game, but how much influence Ronaldo was able to have.
Firstly, United took the lead through a fortuitous own goal from Sergio Ramos. This led to Ronaldo stepping up his efforts to get involved centrally.
Three times in quick succession he received the ball around the edge of the Manchester United penalty area and shifted the ball left and right, looking for the slightest opening in which to aim one of his thunderous shots through.
This was the Ronaldo for whom anything was possible with a "brute force attack" of trying anything enough times and sooner or later, power and repetition will win through.
His chances were routinely blocked; this approach failed to yield dividends.
A change was needed, and he provided.
Nani's dismissal gave Xabi Alonso and, after his introduction as substitute, Luka Modric far more space to operate in centrally. They began pinging passes around the United defensive third and allowing Real to put more emphasis on movement and pace of possession rather than trying to batter down the United door.
After Modric's excellent equaliser, two crosses from Gonzalo Higuain proved decisive.
The first one was fired across inside the six-yard box, but no player in green—Real's away kit—was on the end of it.
It was a golden chance for an easy tap-in, but nobody gambled.
However, Ronaldo has made a habit of scoring close-range goals because of his capacity to enter the penalty area, and the six-yard box, at the right time. So the second Higuain cross-shot saw exactly that happen.
The No. 7's instincts to follow in the ball saw him arrive at the back post in time to slide the ball over the line.
Remembering his former fans, Ronaldo didn't celebrate much, but the significance of his actions was immense: United now needed two goals, with only 10 men, to knock Real Madrid out.
Ronaldo is always a key figure in matches and his willingness to try and try again, and again if necessary, means he will continue to be a threat to the opposition and a match winner on any given day.
Even if he is not at his best.
During the 90 minutes at Old Trafford he attempted no less than 11 shots, a high total even for him—he averages seven shots per game domestically, more than anybody else in Europe's top five leagues.
His constant, ceaseless endeavour always gives his team a chance of finding a goal when they most need it.
Despite being far from his most influential, far from his best and far from the player who gave Manchester United the most trouble on the night, it is still Cristiano Ronaldo who has ended up being the match winner.
With him in the side, Real Madrid still have a shot at Champions League glory this season.