Shaun Maloney's second-half strike earned the Latics their first ever win over the Premier League champions.
"We didn't perform," Giggs told MUTV. "For a start, our normal passing game wasn't as fluent and we kept giving the ball away. Obviously, you've got to give credit to Wigan - they put us under a lot of pressure, especially early on and credit to them as well.
"We just didn't keep the ball like Manchester United usually do. I think we only had one or two shots on goal and that's also disappointing so we have to look at the game and work on it and get ready for Sunday."
United, who now lead Manchester City by five points at the top of the table, host Aston Villa at Old Trafford this weekend.