Stories from Italy have suggested that Chelsea are winning the race to sign the Moroccan international, but the most recent reports believe Louis van Gaal has lodged a big bid to beat his London rivals.
Van Gaal is on the lookout for another defender to fill his preferred 3-4-1-2 formation.
Having missed out on Mats Hummels and Thomas Vermaelen, United have turned their attentions to Benatia after an outstanding campaign last season for Roma.
Benatia was thought to be on the verge of a move to Manchester City earlier this summer, prior to their signing of Frenchman Eliaquim Mangala, and he is now favourites to join their city rivals.
United have also had a bid of £16 million rejected for Sporting Lisbon’s Argentinian international centre-back Marcos Rojo as van Gaal looks to add more depth and experience to his back line.
Van Gaal has grown frustrated at United’s lack of recent movement in the transfer market, having signed Ander Herrera and Luke Shaw early on in the window for a combined fee of £60 million.