Giggs was at fault by way of being rubbish in attack. Look at the second half, how many attacks did Spurs mount in the second vs the third. First part of offence, is up the field, winning the ball and attacking well enough to peg the other team back. Giggs might not have directly made mistakes leading to the goals, but he was part of an offence and probably the weakest/worst part of an offence that led to the other team attacking as much as they did.
Basically the second you switched him, Spurs were pushed back, some of that is mentality of players, tactics in the second half, players trying to hold on to it but a huge part of it was a hugely better offence making it too difficult for Spurs to get out.
Likewise, he also didn't get back to help with either goal, if you went with say Buttner at left back and Evra left wing, maybe the pace or reading of the game of one of those two would mean one of them would have made it back for the second goal.... maybe not.
Also Kagawa did plenty centrally in the second half, if was having more players making runs. RVP didn't do badly in the first half, he just didn't have the ball, one guy up on his own with one guy passing to him and the midfield pushed way back. As soon as the link up play starting happening and Utd players got closer to each other, the passing started to work. Kagawa scored making a run almost infront of the goal and very central. Two guys, tall and heavy or thin and lightweight will have trouble against 6 players with no support, 4 tiny guys won't do badly when up against 3-4 defenders. Defoe did great against Ferdy who towered over him. It's not size or physicality, it was the runs, the support and the options that were just non existant in the first half.
I'll even give Carrick credit, for the first time in really years you could spot Carrick get the ball with his back to goal, with players around him and an easy pass back to the CB, he turned and passed 20 yards forward. I player turning and passing 20 yards forwards rather than passing sideways or backwards is the difference between a fast break and a poor attack against 10 men behind the ball. It's such a small and simple thing but absolutely crucial.