Question one. Does a great footballer need to be good at everything that a footballer can do?
No. Most of the great footballers -- with the possible exception of Johan Cruyff, who was ridiculous -- weren't great at every single thing that a footballer can be great at. Pelé, I'm guessing, couldn't man-mark. Which is fine, because if you've got Pelé in your team, then not only would asking him to do a job on the opposing number 10 be something of a waste of his talent, but it might also amount to an arrestable offence. Not even Rafael Benítez would stoop so low.
Question two. Does a great striker need to be good at everything a striker can do?
Again, no. Thierry Henry was a great striker: lissom, lithe, and occasionally sensuous. Gerd Müller was a great striker, and as well being nicknamed "der Bomber" was also known, at least by his first coach, as "kleines dickes Müller". That's "short, fat, Müller" in slightly-bad German, according to
Wikipedia research. Henry's game was built around pace and penetration; Müller's around standing in the penalty box and waiting for the ball to bounce off his magnificently un-athlete-appropriate arse*. As long as a striker can get the ball into the net, it doesn't matter how. As the really-quite weird saying has it, there's more than one way to skin a cat.
Question three, then. Does a great goalkeeper need to be good at everything a goalkeeper can do?
Goalkeepers have it a little harder than other players. A striker that can't really head the ball, say, or use their left foot, can get by perfectly well, so long as they can compensate by other means. But the last-line-of-defence nature of the goalkeeper means that any inadequacies, if exposed, tend to fall not into the fanciful realm of ‘oh, he should have scored', but in the all-too-real ‘ah, he's conceded'. Fail to jump in the opposition's box, and people will rue that which they never had; fail to jump in your own, and they'll blame you for what they've definitively lost.
All of which brings us, by an admittedly-ambulatory path that suggests that whoever's writing this has never really got the hang of structure, to David de Gea. It's not unfair to suggest that given the money Manchester United spent on the bequiffed Spaniard, they have high hopes of his attaining greatness; or at least, attaining a level of performance adequate for the rarified standards that Manchester United set for themselves, or have set for them by the good people of Nike, Chevrolet and Mister Potato, whose concerns, while not centred on or inspired by footballing greatness, certainly require it or something close to it for their exposure to be positive and their partnerships to be fruitful.
De Gea, then. General Consensus is that he's very, very good at some things, those things being shots and the stopping of. General Consensus is that he's frankly deficient in other areas, particularly crosses, and the dealing with. General Consensus knows lots of things -- it's his military background -- and let's assume he's right. But how important is the fact that De Gea (sometimes, a bit) can't deal with crosses?
On the surface, very. Just as a one-footed winger can be neutralised by showing him onto his weaker side, so a vulnerable goalkeeper can be targeted. Send the ball wide, find the mixer, repeat until he sprawls across the turf like a baby giraffe, take the goal. But there are two points weighing against this significance. The first (and most obvious) is that this cunning plan can be neutralised, or at least lessened, in a variety of ways that don't involve De Gea at all: lessening the number of crosses that come in; working to make sure that those that do come from the weaker members of the opposition; ensuring that opposing strikers are tightly marked, or offside, or otherwise inconvenienced. Defending, they call it. Just as a good back four will try to work to its own strengths and counter its weaknesses, so it should also try to work to and counter those of its goalkeeper.
The second point is more abstract. The question is not: does De Gea's weakness at dealing with crosses cost Manchester United points? It is more complex than that: does De Gea, when you weigh his strengths against his weaknesses, lose or gain points for Manchester United? A definitive answer is beyond me but, thinking anecdotally, it certainly feels like every waft at a cross (that another keeper might have claimed) comes with plenty of stopped shots (that might have been beyond another keeper). A keeper with better handling but lesser reflexes might not have been in any position to gift Tottenham their late equaliser.
Just as whichever club employed Davor Šuker tolerated what he couldn't do with his right foot because of what he could do with his left, so whichever club employs De Gea at the moment has to tolerate the fact that he can flap at crosses. But even without taking account of his age, his potential for improvement, and the tombola in front of him, there's a case to be made that he is already good enough for Manchester United, that what he gives up in those flappy moments he gets back with his reflexes. A goalkeeper can, as a whole, be both rubbish at dealing with crosses and good enough. It's not necessarily an either/or.
Take the game against Real Madrid. His second-half stop with his feet was a remarkable piece of improvised, playground keeping, but his most impressive moment came in the first half, when he tipped a Fábio Coentrão shot onto the post. As well as preserving the 0-0 at a a time when United were wobbling like a drunk jelly, it was a save that left Peter Schmeichel -- who you'd hope knows a thing or two about goalkeeping -- at a loss to comprehend how he'd got to the ball. A goalkeeper that can do things that Peter Schmeichel can't understand? That's got to be worth the occasional wobble in the six-yard box.