We only have 4 first-team defenders (yes I'm counting Michael Keane), and 3 of them are injury-prone. We'll need at least 1 more central defender to come in to make this work. On top of that, we are prone to getting caught out at the wide areas given that our wing backs push up high. It'll take some time for the defenders to get used to this system. Plus, we only have Fletcher and Carrick who demonstrate the discipline and defensive capability to play in this setup. Unless the likes of Cleverley, Herrera, and Fellaini improve defensively, we'll need to get a new midfielder who'll be more defensively solid.
Finally, for this work, we'll need some dynamism and pace up front to stretch the opposition back line and create space in the middle. We'll also need players who are good when they're isolated. One of the strikers must have pace and movement to stretch the opposition, so that will be Wilson, Welbeck, or Chicharito. The other can be anyone else, so that will include Rooney, van Persie, etc. What this means is that we can't play both Rooney and van Persie up front as both aren't that great when they're by themselves, and both don't have the pace to stretch the opposition and keep them on the back foot. Plus, how will Januzaj get the match time to develop? He can play behind the forwards, but Mata's ahead of him. He can play up front, but he has several players ahead of him there.
We still have too many attacking players for the 3 roles up front, and we lack the number of central defenders needed for such a system. We also lack midfielders with good defensive discipline. I still think we may go for a 4-man defensive setup in the end given how few central defenders we have.