United's defenders have been savaged in the media after squandering a 3-1 lead to lose 5-3 at Leicester City on Sunday but Neville, who won six Premier League titles with United,  said he was not surprised at what happened.
"I know United have spent 150 million pounds," Neville told BBC Radio 5 live. "I think there's another two transfer windows of similar amounts of money needed -- maybe 100 million -- before they can even think about winning the title.
"There are still key positions in the team that need filling.
"I think the centre-back position is obviously the key area," Neville added. "Holding midfield or centre midfield needs addressing too."
The imbalance in the United team was glaringly obvious on Sunday with Robin van Persie, Angel di Maria, Ander Herrera, Radamel Falcao and Wayne Rooney looking dangerous going forward.
But United's defending was shambolic, leaving new coach Louis van Gaal needing to fix the problems urgently if United are going to launch any kind of bid for the top four positions, never mind the title, this season.
To add to his problems, Jonny Evans went off injured  leaving the ground on crutches and with his foot in a protective boot, while young defender Tyler Blackett, who has played in all five league matches at centre-half was sent off and will be suspended from Saturday's game against West Ham at Old Trafford.
United signed forward Di Maria, midfielder Herrera, defenders Luke Shaw, Marcos Rojo and Daley Blind in the summer but did not sign a top-class centre back to replace the departed Rio Ferdinand or Nemanja Vidic.
Neville, who was on the coaching staff at United last season, said there was a good reason for that.
"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," he said.
"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."
United have made a worse start to this season than they did a year ago when David Moyes was starting his short-lived stint as manager.
After five games last season United had seven points and were eighth in the table, five points off the lead.
They are currently 12th with five points and one win from five games, leaving them eight points behind early pace-setters Chelsea.
If that was not bad enough, they also crashed to one of their most humiliating defeats when League Two (third tier) side Milton Keynes Dons defeated a team containing David de Gea, Evans, Adnan Januzaj, Anderson and the now departed Danny Welbeck and Javier Hernandez 4-0 in the Capital One (League Cup) a month ago.

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