The Red Devils became the Premier League's biggest summer spenders after paying around £150m to recruit Luke Shaw, Ander Herrera, Marcos Rojo, Angel di Maria, Daley Blind and Radamel Falcao, who joined from Monaco on loan.
Despite bringing in a wealth of attacking talent, United proved that they remain vulnerable in defence having succumbed to a 5-3 defeat against Leicester City on Sunday after having a 3-1 lead at the King Power Stadium.
"I know United have spent £150m," Neville told BBC Radio 5 live. "I think there's another two transfer windows of similar amounts of money needed, maybe £100m, before they can even think about winning the title.
"I think the centre-back position is obviously the key area. Holding midfield or centre-midfield needs addressing [too]."
United have won just once in five league games under manager Louis van Gaal this season.