Gary Neville has admitted Manchester United let themselves down in their stiffest examination of the season so far.
The optimism generated by a four-match winning start to the new Premiership campaign was dramatically punctured in front of a record crowd at Old Trafford yesterday.
Not only were United beaten by Arsenal, they were outplayed in virtually all areas, with only debutant keeper Tomasz Kuszczak emerging with any credit.
For a side who were starting to feel genuinely capable of mounting a stern challenge to Chelsea's hopes of landing a title hat-trick, the manner of defeat was a major body blow.
And Neville, a veteran of six championship-winning campaigns, knows some lessons will have to be learned pretty quickly if the Red Devils are not to endure a fourth consecutive barren year.
'It was the toughest test we have faced so far this season and we failed it,' said the United skipper.
'We definitely let ourselves down with our passing. It wasn't fast enough, crisp enough or accurate enough.
'We have learned a lot from games like this over the past two seasons but we have to stop learning and start putting things into practice.'
Cristiano Ronaldo may have been the obvious villain when he was robbed by Cesc Fabregas in the build-up to the Gunners' late winner but the Portugal winger was far from the only culprit.
John O'Shea had already got away with a worse crime when he passed the ball blindly behind him to Julio Baptista, only for the Brazilian to blast his shot just wide.
And, rather that merely pointing the finger at Ronaldo, Neville wants his team-mates to take collective responsibility for their first league loss to Arsenal since 2002.
'It is not about one player giving the ball away, it is about all of us doing it at one stage or another in areas where Arsenal could capitalise,' said the 31-year-old.
'They may not be as strong as they have been in the past in terms of physical presence but they have players in midfield who will punish you if you give them the ball and we did it too many times.'
With a full week to prepare before Saturday's trip to improving Reading, at least Sir Alex Ferguson will be able to get the tiredness out of his side's legs.
Ferguson claimed the exertions of the midweek Champions League battle with Celtic took its toll in the latter stages yesterday, although, as Neville correctly observed, Arsenal were similarly engaged in Hamburg.
'There is no reason why we should be in a worse state than them but it looked like it at times,' he added.
Of more concern should be yet another reminder that while United have proved adept at disposing of the Premiership's makeweights, flaws in midfield mean they appear far less convincing against their major rivals.
Surely it is more than mere coincidence that their three defeats since the beginning of February have come against Liverpool, Chelsea and now Arsenal.
Although Ferguson put the omission of Michael Carrick down to the need to keep his side fresh for their third game in a week, doubts persist over the £18.6million new boy's ability to forge a successful midfield partnership with Paul Scholes given neither man is particularly adept at defending.
John O'Shea was again exposed as being just short of top class in that department yesterday and never has the need for an Owen Hargreaves-type figure looked so obvious.
Although Hargreaves is now recovering from a broken leg, he should be back in action well before the January transfer window opens and a renewed bid from Ferguson is expected.
United can only hope they are still in the title picture by then although, unlike last season, they can at least console themselves with the knowledge that the other three participants in Grand Slam Sunday have lost at some stage this season as well.
'We must not get downhearted by this result because I believe the league will be much tighter this season,' said Neville.
'We have to make sure we keep in touch with the top and that means winning at Reading on Saturday.
'At this club, when you lose, you must make sure you recover quickly, then go on another run.'
Meanwhile, United have received some good news with confirmation Park Ji-sung's ankle operation has been a success.
The South Korean will be out of action for three months after damaging ligaments in last weekend's win over Tottenham.
But the early indications are the 25-year-old is on track to recover as scheduled in time for the packed Christmas programme.
'The operation went well,' Park told the Chosun Ilbo newspaper.
'I feel frustrated that I won't be able to play for about three months but I am confident after the operation I will be stronger than before.'