Sebastian Larsson struck the only goal of the game in the 30th minute, which ensured that Giggs suffered the first loss of his fledgling managerial reign.
"I feel flat," Giggs told BBC Sport. "The players were flat and I don't know why. Credit to Sunderland. They are in good form and they created the best chances. We lacked that bit of quality in the final third.
"We were putting five or 10-yard passes astray and if you do that in the Premier League you get punished. We've had a problem at home all season. It's not been good enough.
"I can't tell you anything about the next manager. I am just concentrating on the next game. The quality is still there in the dressing room but we didn't show it."
The result means that Sunderland have now won their last three Premier League games.