Jose Mourinho has already spent close to £150million on Nemanja Matic, Romelu Lukaku and Victor Lindelof.
The Portuguese tactician has identified a winger as a priority and has been chasing Inter Milan's Ivan Perisic and Real Madrid star Gareth Bale.
However, Bale looks set to stay at the Bernabeu, while a deal for Perisic has dragged on the entire summer without much progress.
And Neville is convinced that a winger is the missing piece of the jigsaw for the Red Devils, as it is not Marcus Rashford's favoured position.