With Romelu Lukaku on the verge of joining Inter Milan, United have identified the Athletic Club striker as a potential replacement.
Athletic's Basque-only recruitment policy tends to place a premium on their players, and they have told United that they will not sell the 25-year-old for less than his buyout clause.
But that has not necessarily killed the deal, with the Red Devils reportedly open to paying £81m for the frontman.
Williams scored 14 goals in 41 appearances in all competitions last season.
He has been capped once by Spain in a friendly against Norway, but could yet switch his international allegiance to Ghana.
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.