There is no way such a midfield will ever, ever occur. Chelsea are never gonna sell Mata to a rival. Garay and Denswil are not good enough. Vidal *could* be an option, but an expensive one. Koke most probably won't leave, now the WC is so near. Plus, expensive buyout clause in the new contract. Reus, meh. He won't move to United.
Alex Sandro for left, maybe Barkley (seems unrealistic, Everton won't sell, and United don't have that much buying power) or Ward-Prowse for midfield for the future (Lallana quashed a move to United);; Everton Ribeiro would have been a decent option, but he doesn't want to move.. We don't really need another forward upfront, atleast until Summer (Provided Hernandez isn't sold to Arsenal).
Pogba would be an excellent midfield option to get back, but he won't want to move back to United T_T Ravel Morrison seems like a more realistic choice though..
With van Persie's injuries, Rooney can move upfront (Once he recovers from groin injuries), with Kagawa flitting into the AMC role, where he was deadly for BVB. He's been used in different positions, thus never really contributing to United.