All 48,000 tickets sold out in 27 minutes, the majority to United fans who will be hoping the match is more entertaining than the club's four pre-season games so far. United have scored just three goals, but pre-season results count for little, with the players seeing it as a chance to ease their way back towards full fitness. Any professional will tell you that they don't reach that level until six weeks into the season, but if there's pressure on United to start the season well, it is nothing compared with what's expected of Barcelona.
Wednesday will be the biggest test so far for new Barca coach Tito Vilanova and the 42-year-old Catalan will have little time to impress.
Four years ago, Pep Guardiola took charge with Vilanova at his side. Guardiola was established as a Barca legend and a hugely underrated player outside of Spain, yet he got off to an uneasy coaching start with a defeat at a Numancia side who would finish bottom. A 1-1 home draw against Racing Santander followed and those two results, those four points dropped, were enough to cast doubts on Guardiola's suitability.
It's not something Barca fans like to be reminded of now for there was little love for Guardiola as Barca played their third game under him, with critics pointing out that the crowd had dropped below 60,000 for the first time in five years. The small crowd booed too — though the objectors were more against the president Joan Laporta than Guardiola.
Such protests melted away as Barca won 19 and drew one of their next 20 league games, building up an unassailable lead which helped them win the league. Vilanova explained then how important it was to make the fans proud of their team again. "Week by week, we managed to lure people back, a thousand here, a thousand there," he said. A tactician who paid attention to the tiniest of details when looking at opponents, Vilanova was so integral to the success that he was right to use the term "we".
The good form wasn't confined to the league. Barca won every competition they entered in Guardiola's first season, but the early concerns highlighted how short-burning the managerial fuse is when you are in charge at Camp Nou.
Vilanova is still in what could be a very short honeymoon period. The players talk of their respect and their positive relationship with someone they know well having worked with him on a daily basis for four years. Some, like Pedro, have worked with him even longer as he was also by Guardiola's side at Barca B in 2007-08. So far, so good then, but the mood will shift sharply if Barca are not as good as last season. His side have to be viewed as the best in the world from the start and failure will be Barca not winning either the league or the Champions League by playing beautiful football.
It's a big ask, especially as Vilanova is up against the best Real Madrid team in a decade, a team coached by someone with far more experience in management and in the media, someone who poked Vilanova in the eye during last season's heated Spanish Super Cup. The same two teams meet in the same competition later this month.
I spoke to a player who has worked with Vilanova last week and he said: "Very nice guy who is serious about his football. He is big into preparation, the team philosophy and he knows Barca inside out, but it's a huge step up for him. He doesn't have the respect, force of personality or legendary status which Guardiola had built up as a player before he took charge.
"The players will say they like him now, but they will be the first to undermine him if things don't go well. They are used to him being friendly which is what assistant coaches must be — a conduit between the coach and the players. He now has to make difficult and sometimes unpopular decisions. At any time, a player can turn around and say: 'And how would you know? You never made it as a top level player.'
"Players look after themselves first and though it was never said, Guardiola had to wrestle with some huge egos last season in that Barca dressing room. Those players — and there are three or four of them — will be the first to complain if things don't go well."
Vilanova was a surprise appointment after Guardiola stepped down. There were gasps when his name was read out to players and the media, but he was appointed with Guardiola's blessing, the boot room philosophy of promoting within favoured.
Vilanova and Guardiola are both Catalans who came through Barca's La Masia as young footballers and Vilanova's own backroom staff will be packed with alumni from that same famed academy.
The former midfielder inherits a settled squad which he knows well. The only signing so far has been left-back Jordi Alba from Valencia, a former Masia student himself. Alba is a Catalan who grew up close to Camp Nou, but he had to leave Catalonia to get his break. He's now Spain's best left-back.
There may be more arrivals — a midfielder to replace the outgoing Seydou Keita for a start. Athletic Club's Javi Martinez was the preferred option and he can also play in defence, but the Basques want £34 million (Barca's entire annual transfer budget) and the Catalans have baulked at his wage demands. Cash-rich Athletic are never under pressure to sell any player, especially their captain. That leaves Alex Song, though Arsenal value him at £20m and Barca £12m.
Barca are also keen on Liverpool's Daniel Agger to provide defensive cover, while talk of the Brazilian Neymar coming to Camp Nou next season continues in newspapers desperate for summer news.
David Villa's return will feel like a new signing and the Asturian is expected to play in Gothenburg - or Gotham City to Bryan Robson.
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