Furious Manchester United boss Sir Alex Ferguson branded his side's 4-1 defeat to Middlesbrough an "embarrassment".
The error-ridden visitors were 3-0 down at half-time as they suffered their heaviest defeat for 19 months.
"It was a shocking performance," said Ferguson. "We conceded some terrible goals and we can't afford do that with what we're trying to achieve."
United are now sixth following their second defeat of the season, 13 points adrift of Premiership leaders Chelsea.
Ferguson was particularly unimpressed with his defence.
"On that form, we couldn't beat anybody. Defensively, we were very disappointing," said the Scot.
"But we will get the right response all right, that is without question."
United's misery began in the second minute when Gaizka Mendieta's long-range shot slipped through Edwin van der Sar's fingers.
Rio Ferdinand was culpable for the second and fourth goals in what was a nightmare performance from the £29.1m England star, who was eventually replaced by Wes Brown.
Kieran Richardson was also at fault, conceding the penalty which allowed Aiyegbeni Yakubu to stroke home the third.
"We conceded in the second minute to a free shot from 25 yards and we never recovered from that," said Ferguson.
"The second and third goals were terrible ones to lose. You can't afford to do that at this level."
Only Cristiano Ronaldo's injury time header - ironically the club's 1,000th Premiership goal - prevented the result from being their heaviest since 1999.
However, BBC Sport pundit Lee Dixon told Match of the Day that he felt that this was United's worst display in recent memory.
"They have played badly but not this badly," said the former Arsenal defender.
"Every department of their game was off from start to finish and they were lucky in the end to come off 4-1 losers."
United's next outing is against Lille in the Champions League on Wednesday, followed by an Old Trafford showdown with Chelsea next weekend.