Sir Alex Ferguson told his players to expect a kick up the backside but not even he could have forecast a 4-1 humiliation and potentially pivotal defeat by fiercest rivals Liverpool on Saturday.
The Merseysiders backed up their hammering of Real Madrid in the Champions League on Tuesday by inflicting United's biggest home reverse since 1992 to breathe new life into their Premier League title challenge.
The wise money remains on Ferguson's side to secure their third successive crown but with tricky home games against Aston Villa, Manchester City and Arsenal still to negotiate, Rafa Benitez's men have ensured a fight to the winning post.
Ferguson did himself no favours when he went against 75,569 people inside Old Trafford to reflect with a vaguely spiteful and blind assertion that United "were the better team" but it is difficult to see any clarity to his argument.
The nature of this shock result was made all the more remarkable given that Cristiano Ronaldo had put the home side ahead from the penalty spot after Pepe Reina had sent Park Ji-Sung tumbling.
Liverpool's riposte, however, was swift and savage.
Yet for all their attacking verve and intent, United had a hand in their own downfall courtesy of some calamitous and uncharacteristically bad defending.
Nemanja Vidic endured his worst day in United colours and inexplicably allowed a routine long ball to bounce for Fernando Torres to pounce for the equaliser, before Patrice Evra brought down Steven Gerrard for the Liverpool skipper to put the visitors ahead from the penalty spot.
Vidic suffered his second red card against Liverpool this season when he hauled down Gerrard and was still making his way down the tunnel when Fabio Aurelio curled home the visitors' third - only for Andrea Dossena to make the scoreline more emphatic with an exquisite lob.
For Liverpool it was the perfect end to a day which could yet come back to haunt Ferguson.