United lacked tempo, their passing was poor and they could not find a way through a defence that has shipped 57 goals this season.
The feel-good bubble that surrounded Old Trafford following the interim appointment of Giggs and his fellow Class of '92 graduates has now burst.
"I feel flat, the players were a bit flat, and I don't know why," the Welshman said.
"We controlled possession but we just lacked that quality in the final third, and concentration."
United have now lost seven times at Old Trafford this year. Stoke, West Brom and Norwich have better home records.
For a man who is used to winning game after game at Old Trafford, Giggs admits such a poor record is hard to take.
"We have shown good form away from home but at Old Trafford seems to have been a problem," the 40-year-old said.
"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."
Giggs, who is out of contract in the summer, has yet to decide whether he will play in Tuesday's match against Hull.
With Louis van Gaal set to be appointed as David Moyes' successor, there is no guarantee that Giggs will be around next year.
Wayne Rooney may take to the field on Tuesday after he was left out against Sunderland due to a groin problem, though.
"We will keep monitoring him and see how he is for Tuesday," Giggs said.
"Hopefully he will be back."
Meanwhile Sunderland's "miraculous" great escape is almost complete after they clinched their first win at Manchester United for almost half a century according to Gus Poyet.
"This is close to a miracle," the Sunderland manager said.
"A miracle was the word I used a few weeks ago and apparently every now and again it happens.
"We thought we would have to win four matches after losing to Everton (on April 12).
"We won three and if we win one (more) we are over the line. Chelsea could help us tomorrow. I don't know if I will be able to watch.
"What we have done in the last four games is unique.
"It is special, incredible...
"We were getting no breaks. Normally that sends you down, and then you need something very special."
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