It is not just the occasion - the ground's first European Champions' Cup quarter-final since the 1960s - but the timing which has generated the tingle of anticipation.
Three months ago, when Manchester United were paired with Porto, there was a sense of foreboding at Old Trafford. By their standards, United were languishing in the Premiership and fortunate to have survived in Europe. Porto were already on course for a third successive domestic title and, in Europe, they had humbled Milan.
But a cleansing winter in Manchester has been matched by a murky one in Porto. Even concern over an ankle injury to Roy Keane cannot dim the dreams of a United team which is now four points clear in the Premiership. Porto, although even further clear in their league, have become assailed by fears.
Against weak opposition, the team have taken one point from the last six, but that is the least of their worries. The club itself is facing disgrace.
The last thing Porto's chairman, Jorge Pinto da Costa, wanted to see when he arrived in England this week was headlines about match-rigging. He has enough of those at home where he is at the centre of a scandal.
Briefly, Da Costa, who recently resigned as the head of the body which appoints Portuguese referees, is accused of systematically bribing and intimidating the officials. One of the allegations, all of which he denies, concerns a holiday. It was paid for by the club, arranged by the travel agent brother of Antonio Oliveira, the current Porto manager, and taken by a referee.
There is also a European angle, a piquant one in the circumstances. Da Costa has been accused, by the president of another Portuguese club, of bribing the Romanian referee of another European quarter-final, 12 years ago, in the European Cup-Winners' Cup. The opponents were Aberdeen, the manager Alex Ferguson. Porto won.
Uefa, the sport's European governing body, has taken an interest in the allegations and in Portugal there is such disenchantment with the game one group of prominent intellectuals renounced Portuguese football and declared they would henceforth follow Barcelona.
This is the backdrop to Porto's arrival in Manchester. Yet they cannot be underestimated. Their players are capable of winning without any help from referees. One key figure is Mario Jardel, the Brazilian who would be at Ibrox had Rangers managed to secure a work permit. Quick with his feet and powerful in the air, he is the Portuguese League's leading scorer and struck twice in the San Siro when Porto beat Milan.
The other central figure is Jorge Costa, who achieved international notoriety when he was head-butted by George Weah in the group stage. Costa is suing Weah for alleging the assault followed racist provocation. Costa does not deny being a firm tackler and his encounter with Eric Cantona is potentially explosive.
There is a Slovenian, Zlatko Zahovic, a Yugoslav, Ljubinko Drulovic, and a clutch of other Brazilians. They include Artur, who matched Jardel's four Champions' League goals, Aloisio, Lula, and Edmilson. There could have been a sixth Brazilian on show, but Emerson left Porto for Middlesbrough in the summer. One wonders what he is thinking as he prepares, instead, to play Derby in tonight's relegation scrap at the Riverside.
Keane, whose consistency shames Emerson, is "doubtful" Ferguson said after his fitness test yesterday. "I hoped he would have done better," Ferguson said. "It did not go as well as we wanted. We will given him every chance."
With Nicky Butt already ruled out, the feeling is that Keane will start the first leg with Ronny Johnsen alongside, while, up front, Ole Gunnar Solskjaer will be preferred to Andy Cole.
Ferguson has reminded his team that in Europe opponents can be at their most dangerous when the ball is near their own goal. The lesson appears to be sinking in, with Ryan Giggs saying: "A lot of the European sides like Porto are counter- attacking teams, so you have got to be careful about giving the ball away."
The last Old Trafford European Cup quarter-final was against Rapid Vienna in 1968-69. United, defending the title they had won against another Portuguese side, Benfica, won 3-0. A repeat is unlikely, but if United can keep the tempo high and the back door shut, they should travel to Oporto in a fortnight with something to defend.
Manchester United (probable): Schmeichel; G Neville, Pallister, May, Irwin, Beckham, Johnsen, Keane, Giggs, Cantona, Solskjaer.
Porto (probable): Hilario; Sergio Conceicao, Jorge Costa, Aloisio, Lula, Paulinho Santos, Edmilson, Zahovic, Artur, Drulovic, Jardel.