Seven players have arrived under Jose Mourinho's stewardship and he has admitted he would like and eighth before the window closes on August 31.
If the Portuguese can get a wide man in, Neville thinks his old side will have a top squad.
He said: “Is Rashford a wide player – not really?
“That is where United could get one world class winger and it would be a complete squad.
“They have to challenge this year.
“Last season was not good enough and it was a big under performance from the players."