After surviving a second-half siege as the Portuguese League leaders desperately tried to claw back the advantage Paul Scholes had given the hosts with his 31st-minute header, the hosts’ notoriously fragile defence gave way in the final seconds.

The end came when Phil Neville fouled Edgar Jankauskas on the edge of the area and Benni McCarthy, scorer of both Porto’s goals in the first leg, sent the free-kick arrowing towards the top corner.

Tim Howard clawed the ball away but Francisco Costinha reacted fastest and lashed the the rebound into an empty net to send United spinning out of the competition before the quarter-final stage for the first time in eight years.

Nine points adrift of Barclaycard Premiership leaders Arsenal, it leaves United with only the consolation prize of the FA Cup to play for – and how they will lament the decision to incorrectly rule out a second Scholes goal for offside on the brink of half-time.

Amid all the pre-match acrimony over Ferguson’s belief that the Porto players, talented as they may be, fall to the floor rather too easily, the one thing the Scot had not bothered to stress was the need for patience.

It was a lesson learned early in United’s European adventures so, when the half hour mark passed with Porto dominating possession without United having so much as a single effort on goal, no-one in the home camp, players, coaches or supporters, panicked.

Instead they reserved their ire for the Porto player who more than anyone lived up to the stereotypical image Ferguson had conjured up.

Carlos Alberto may be nothing more than a raw teenager but he clearly learns quickly. It seemed as though every time a United player went near him, he ended up rolling around on the floor.

On each occasion, the reaction won him a free-kick, except that is until he went down in the penalty box under pressure from Wes Brown and Russian referee Valentin Ivanov waved away the penalty claims.

By that stage, Scholes had broken the deadlock, John O’Shea playing a one-two with Ryan Giggs, then dropping a perfect near-post cross onto Scholes’ head which the England international’s ferocious finish left Vitor Baia with no chance.

The goal brought Porto out of their shell a little bit and an Eric Djemba-Djemba mistake offered Alberto a clear shooting chance.

However, the youngster lacked the finishing prowess of the brilliant Brazilian skipper of the same name and he blasted his shot at Tim Howard’s legs.

It was a mistake which should have been punished by a second goal for the hosts but when Scholes stuck out a foot to stop Ryan Giggs’ wayward shot, then slid the ball past Baia and into the net, the nearside assistant referee slowly raised his flag to indicate offside.

TV replays rather more speedily showed he was wrong.

United went into the break knowing they would advance to the last eight if there was no further score but, given the lack of key men, Ferguson understandably assumed he could not rely on his defence to hold the visitors at bay and introduced Louis Saha for Djemba-Djemba, deciding the French striker’s Achilles problem could stand 45 minutes of action.

It was Porto though who carved out the first chance of the second period and Brown did well to get a firm block on Maniche’s goalbound effort.

Carlos Alberto sent another effort trickling by the post which Howard was not entirely sure he had covered and as United were pushed further back, Scholes produced the studs first challenge on Pedros Emanuel which would have ruled him out of the quarter-final first leg if United had got that far.

A dipping volley from Dmitri Alenitchev gave the home side a foretaste of what was to come when it dropped inches over the bar.

Ferguson tried to stem the flow of Porto attacks by introducing Cristiano Ronaldo but it proved to be the briefest of appearances as he was almost instantly caught by Nuno Valente.

Ivanov decided the young Portuguese international had made a meal of it. The sight of Ronaldo being carted to the dressing room on a stretcher indicated otherwise as Ole Gunnar Solskjaer came on in his place.

The battle continued and as the game ticked into its final minute, it seemed as though United would edge through. It only made the killer blow that much harder for United fans to take.