Keeper's aren't perfect, ever, get over it people. Take the Spurs game, he made a bunch of great saves, the points were dropped because Carrick/Jones did entirely nothing in midfield, Evra was inept in dealing with Lennon(one of the reasons Carrick was so frequently deeper than he should be, covering for a crap crap left back).
EVERY KEEPER ever, in the history of football, has cost their team points. You've won title after title with keepers who make constant mistakes, Bartez, Reina, Lehman(he really was very good for a few years), Seaman, Friedal, Given, Casillas, Ceasar, Valdes, and the list goes on. Utd fans(and others) are harping on as if there are perfect keepers out there who are mistake free and De Gea makes mistakes so will never be close to the best around.
Basically, shut up people, keepers, defenders, midfielders, strikers all make mistakes, no one has a 1:1 goals to shots ratio, no one has a 100% pass completion ratio, no one has a 100% tackling success ratio, no keeper makes every save. He's an exceptional shot stopper, end of, thats it, everything else at that point is a bonus. He may be close to the best shot stopper in the game, you don't need to nitpick over his supposed weakness(as if there is a keeper who is perfect at getting to crosses... there is NOT), adapt your game to minimise his weakness. Have your defenders show wingers inside so they are less likely to cross, have two big CB's make a path for him to come out and grab it.
Football is about playing to a teams strengths and minimising the weaknesses. If you had a proper left back rather than the ever absent Evra, you've have less crosses from that side to contend with.
A better central midfield and left back would have reduced the number of chances Spurs had, not a new keeper. 25 shots, the goal was coming, the reason was the pressure and domination of Spurs because Utd sucked elsewhere, not in goal.
Most importantly, there are few if any better keepers, and even more importantly, I couldn't name a keeper I consider better who is 1, available, and 2, not old enough that it would be a fairly short term fix. You've got one of the best shot stoppers in the game, and he's got potentially 15 years left in his career... go out and buy a 30+ keeper from a team that doesn't want to sell for an obscene amount, or with 15 years to go, improve parts of his game and adapt the team to minimise his weakness, which makes more sense? How much would it take to get Casillas off Real, and how much longer would he play, how much would he want in wages and would he come to England, the answers are, buttloads, not that long, buttloads and no.
A better left back and a better defensive midfielder are much more important, and will help out more than a better keeper. Also worth noting is that, without knowing the stat of longest run he's had with the same CB partnership infront of him... I'm going to assume its both low, and assume that the Rio + Vidic combo has played very infrequently infront of De Gea. Better CB partnership, the keeper can better read when to come and when not to, when its a different pair every other week, and different CB's play differently, and the same CB will play a different role depending on which CB he's partnered with... a more settled back four would also improve De Gea's game as would quite obviously, Fergie picking a number one and sticking with it.