I understand why LVG is doing this; We have 6 better attacking central players than a single proper wide player (RVP, Mata, Rooney, Kagawa, Welbeck, Hernandez) right now. The Dutch played 4-3-3 against weaker teams and 3 at the back against stronger ones. If we finish top 4 this year and can go into next season (and the CL) with the ability to play both, we'll have a much better chance to win big games, and against smaller teams, the talent should take care of itself.
I do think this makes us slightly less likely to win the league, but more likely to finish top 4. It's got lower upside, but it's safer in that the group of players is better suited to it.
However, the basic concern that against any team which plays one up front, we have our wide CB's up against pacey strikers in direct confrontations, is a real one. We'll need at least one CB who is comfortable playing wider and has the quickness to do it. Not many of them around. The alternative is that we sign Rodriguez and Shaw moves to LCB for the season, though that's maybe asking a lot of a very young player. But I don't think our defenders really have the mobility + on the ball ability combination needed for the 2 wide CB roles apart from Jones, who gets lost the easiest and who, it's worth noting, started in the middle against the Galaxy. So, I'll feel better about this if we bring in a proper wide CB.
The other major worry is that it leaves fullbacks open to hit deep crosses, and so against mediocre Prem teams with a decent aerial target up front and fullbacks who can cross, we might have a bit more trouble.
Januzaj will get time like Depay did, coming on to provide width, or he'll continue to develop and force his way into a central 3 position and give us some extra width there.
I figure we'll do this for a year, play 4-3-3 fairly often and then by next summer, it'll be mainly 4 at the back with 3 at the back as a sometimes option.
For the next month, the major questions will be:
1) Obviously, how we look playing 3 at the back. If we get torn apart against proper teams (Roma have some real wingers), maybe he'll re-evaluate.
2) Can Cleverley, or less likely Fletcher, be trusted as Herrera's partner until Carrick returns? Do we need another CM, even if it means a rush signing with Vidal seemingly unlikely? If Cleverley plays as the 6, can Kagawa be the 8, or only Mata's backup?
3) Is Shaw going to play LWB, in which case we need to sign a LCB, or should he move inside and we'll sign Rodriguez or another LWB? Seems unlikely we'd spend 60 million in a summer on left-backs, but it's plausible now. I thought, contrary to everyone here seemingly, that Shaw looked good out wide, he just lacked the openings to attack. He can also be pushed to play more aggressively, I'm sure. Technically, I think he definitely has the skills.
4) Which of Keane, Blackett, Thorpe (and now) Fletcher, will be in the lead for 5th CB (I'm assuming we sign a 4th or move Shaw inside)?
5) Can Valencia overtake Rafael as RWB?
6) Which of Nani, Young and Zaha will be sticking around as the other wing option with Januzaj when we go 4-3-3?
7) The pecking order of Evans, Jones, Smalling and figuring out which one plays centrally.
TLDR: 3 at the back mostly this year, though not against dross playing one up front. We don't win the league but we get top 4 and next year, we can sign another player or two and we mostly go 4 at the back but use 3 at the back against top teams and in the CL often. Signings will thus have to fit both formations. That means a pacey LCB (or LWB if Shaw shifts) and a holding CM to pair with Herrera (the case anyways) this summer.