I think we can play it, but I really think we need the directness of Luke Shaw and Valencia/Rafael on the wings. It's our only chance of getting behind the defence unless Welbeck is one of the strikers.
We missed a trick today as when van Persie and Welbeck are not fit, what is the reason to still play 3412? The point of the system is to get our best three players on the pitch (or Welbeck for more pace and defence stretching if one of them is out). Easy to say in hindsight, but we could have played with:
de Gea
Jones Smalling Blackett James
Fletcher Herrera
Lingard Mata Young
Rooney
That was only dropping Hernandez from today's line-up, which a lot of people were indeed calling for in this system. It's also not starting any of the supposedly unfit players (Januzaj, Fellaini, Nani) even though I do think the two wider players could have started in this system and been subbed off instead if they tired.
I do think we should persevere with 3412, but the reason LvG is playing it is to get our best players on the pitch.. Today, to do that he didn't need to play 3412. I can understand why he did (continuity, everyone getting used to the system in the Premier League etc) but he changed it at half time anyway, so maybe he should have thought about that before the game - especially with the pace Swansea had on the flanks.