I really think and have been pretty vocal about it, I really think we could play 3 at the back. Now I know the challenges it brings but hear me out. Evans, Jones and Smalling are all CB's, all good on the ball, Evans obviously the strongest. Rafael and I would opt for Fabio but any attacking LB (Baines?). I really think it would work, or at least worth a try. Something like this.
-De Gea-
-Jones- -Smalling- -Evans-
-Rafael------------------------------Fabio-
-Carrick- -Cleverley-
-Kagawa-
-Rooney- -RVP-
With this, you have 3 strong central defenders who can play. Two midfielders who can receive the ball and play out. You have width with the twins. When the ball is on our right, Fabio drops in and makes 4 at the back and the same goes the other side, if Fabio is pressing the ball on the left, Rafael comes in at RB.
It enables Kagawa to be used in his best position and keeps Rooney and RVP in the side.
Again, I know the issues that this system has but I think we have the players that could pull it off.