Former Manchester United defender Phil Neville believes Louis van Gaal's summer £150m overhaul was not enough, claiming the Premier League giants need to spend at least another £100m in the next two transfer windows to challenge for titles again.
Van Gaal spent more than £150m signing Ander Herrera, Luke Shaw, Marcos Rojo, Angel Di Maria, Daley Blind and on-loan Radamel Falcao.
However, the Dutch manager failed to secure a top centre-back to fill the gap left by both Rio Ferdinand and Nemanja Vidic, who left when their contracts at Old Trafford expired.
"I know United have spent £150m. 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," Neville told BBC Radio 5 live.
"There are still key positions in the team that need filling. I think the centre-back position is obviously the key area. Holding midfield or centre midfield needs addressing [too]."
The van Gaal era at Old Trafford has actually begun even more badly than David Moyes' tenure.
United have only secured five points from the first five Premier League games (one victory, two draws and two defeats), and were eliminated from the League Cup after a humiliating 4-0 defeat against Milton Keynes.
Having already conceded eight goals in the Premier League, Neville addressed the defensive problems by saying: "That squad needed quality, not just run-of-the-mill players, they needed world-class players, and there were no world-class centre-backs out there.
"I think that's why in the next two transfer windows, United might still be looking for that world-class defender.
"I am certain Chris Smalling will come good and I have full faith in Phil Jones as well. They've bought a lot of foreign players, you don't get rid of your English spine."