The former Southampton left-back—signed for a whopping, initial £27 million this summer—has returned from a two-week break following the 2014 FIFA World Cup slower and less industrious than the boss would like.
"He needs to be fit and is not fit enough to do what I want. He needs to train individually until he is fit," Van Gaal told reporters in the United States (via Jamie Jackson of The Guardian). "How long? I have heard good messages but we have to see."
With Rafael returning home from the USA tour with a groin issue and a lack of cover for the wing-back positions anyway, this is a problem for LVG. We do not, however, know the extent of Shaw's issues, and there's a strong chance he simply needs to be fitter than everyone else.
That Van Gaal explicitly states Shaw is not fit enough "to do what I want" is a big clue.
There is such a thing as a World Cup fitness hangover, but Shaw isn't the type to suffer. He proved over the course of 35 Premier League starts in 2013-14 that he was one of, if not the, fittest player in the Saints setup, regularly flying up and down the wing to offer impact in every third.
With Van Gaal confirming a switch to 3-4-1-2 formation, Shaw's role in the team becomes massive; structurally more important than Patrice Evra's was in the entire eight years he spent at Old Trafford.
There's every chance he's not unfit at all—he's just required to run 2-3,000 metres more than anyone else in every game. A glance at LVG's Netherlands system in Brazil would confirm that, and a glance at Shaw's duties under Mauricio Pochettino would support the notion he can play the role.
In the 3-4-1-2 (3-5-2), there's only one "true" wide player on each side: the wing-back. He's tasked with maintaining the width for his side—stretching the pitch in the process—and he receives no help from a winger.
In a 4-3-3, 4-2-3-1, 3-4-3 and any other common formation, a full-back and a winger combine on the touchline. It shares the burden and allows two to work together to win the wide territory and drag their side up the pitch.
We've seen the likes of Juventus, playing a 3-5-2 under Antonio Conte, fail to spring Stephan Lichtsteiner and Federico Peluso up the pitch, and therefore struggle to work the ball forward. If the wing-backs lose their individual battle, the team will nearly always lose the broader one.
LVG wants an uber-fit Shaw on the left to ensure he never loses the territorial battle on that side; he will always have an option to pass to high up and surge forward, winning his side ground.
With very little cover and little chance of signing it, the onus is on Shaw to carry the load. Van Gaal has admitted he's finding roles for out-of-position players such as Wilfried Zaha and Ashley Young, and the latter seems to be the choice to play as a reserve wing-back.
The 3-4-1-2 hinges on the wing-backs—it's arguably the most important position in the formation—and Van Gaal is being studious in his preparations. An uber-fit Shaw signals success; a regular Shaw jeopardises everything.