Any hope of the Black Cats following up their crucial win over Reading by ending a barren run at Manchester United that stretches back to 1968 was effectively ended when they fell two goals behind after just 20 minutes.
Although Sunderland showed spirited resistance in the final half hour, Frazier Campbell's header was all O'Neill's men had to show for a 3-1 defeat that leaves them just a point above the drop zone.
It is not a comfortable position to be in - and with Christmas encounters with Manchester City and Tottenham looming, next Saturday's trip to Southampton is taking on `must-win' proportions.
"It is almost inevitable and pretty natural that when a side is trying to put a bit of form together, confidence is essential," said O'Neill.
"We scored an early goal against Reading on Tuesday and you could see the confidence coming right back to the team.
"But if you concede two goals against anybody, particularly away from home, you will have an uphill struggle.
"Against Manchester United we have made it much easier than we could have done."
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- Manchester United