Such a signing will not make any sense as I feel Herrera, Blind and mata are a good starting midfield trio/duo. However, if Van Gaal feels that we need a subsidiary midfielder to play under Blind and Herrera. I.e., Kovacic, schnejderlin or Youri tieleman, then that will perhaps make more sense than going after a star midfielder like Pogba.
I would think United will be searching for a #8 to replace Fellaini in the first XI; someone with greater ability on the ball, while still retaining some measure of the Belgian's physicality (Pogba would probably be the ideal choice, Vidal, Nainggolan, Xhaka, A. Gomes, Parejo, L. Bender might also be among those considered I suppose. Perhaps Verratti or Koke for sheer quality).