Ryan Giggs admitted he was at a loss to explain why Manchester United produced such a "flat" performance as Sunderland won at Old Trafford for the first time since 1968.
United slumped to a first defeat of interim manager Giggs' spell in caretaker charge after Sebastian Larsson scored a 30th minute winner that took the Black Cats to the brink of securing safety.
Giggs, who had beaten Norwich 4-0 in his first game in charge, was bemused by United's poor display against Sunderland.
"I still believe there is quality in the dressing room," he said in a news conference. "We showed that last week but just didn't this. The players were flat and I don't know why."
"In the first half we got some decent crosses in, and just didn't get enough people in the box. In the second half we had all the possession and just couldn't find that key pass or bit of quality to get us back in the game. I am sure if we scored one we would have gone on to score more."
It was United's seventh home league defeat of the season, a fate they had not suffered since 1973-74, when they were relegated, and Giggs could not account for their struggles at Old Trafford.
"We have shown good form away from home but at Old Trafford it seems to have been a problem," he added. "In past seasons, there would be waves and waves of shots and saves and you could always sense a goal was coming but this season it doesn't seem to have happened."
Sunderland have taken 10 points from their last four games, including a draw away at Manchester City and wins at Chelsea and United and manager Gus Poyet hailed them for a "unique" achievement after he claimed they had needed a miracle.
"What we have done in the last four games is unique, incredible," he said. "A miracle was the word I used a few weeks ago and apparently every now and again it happens."
Given their goal difference, Sunderland, who still have home games against West Brom and Swansea, could be safe on Sunday, should Norwich lose to Chelsea.
"Now it will be a shame if we don't stay up," Poyet added. "It would be crazy to lose this opportunity."