Occasionally the game of football throws up a scoreline that triggers a memory that had seemingly been forgotten.
With goals from Aguero, Dzeko, Silva and Pizarro, City eased into the next round winning 6-1 on aggregate.
Porto have not enjoyed many good nights in Engand through the years, their best was a 1-1 draw at Old Trafford under Jose Mourinho which knocked United out of the Champions League in 2004, thanks to their 2-1 first leg win in Portugal.
But last night's game will have brought back nightmare memories for Porto fans of an equally unsuccessful trip to Manchester back in 1997.
It was a Champions League quarter final, and they went there in high hopes, despite missing star striker Jardel through injury.
The side featured fellow Brazilians Edmilson and Flavio Conceicao, with Zlatko Zahovic the danger man on the wing. They had a big billing, accumulating more points in the group stages than any other club, but never lived up to it in the knockout stages.
United were just hitting their stride at the time, having secured a domestic double the year before, and were en route to winning a second successive Premier League title.
It was one of the best ever performances from the United team of that era in Europe.
The team lined up like this, Schmeichel, May, Pallister, Irwin, Neville, Beckham, Johnsen, Giggs, Cantona, Cole, Solskjaer.
It was David May who bagged the first goal, netting from a corner, before the always controversial Frenchman Eric Cantona scored a second.
The second half was when United took control, with a superb pass from Cantona setting up Giggs who had run 50 yards to rifle in at the near post.
Andy Cole added a fourth, putting United in cruise control heading into the second leg which they saw out 0-0.
It was United's first big statement of intent in Europe after their 4-0 hammering at the Nou Camp three years before.
The win took them into the semi-finals where they lost to eventual winners Borussia Dortmund.
United fans what do you remember about this night?
image: © tEdits