United – who finished seventh last term – embarked on a lavish spending spree in the transfer window, parting with more than £150 million in a desperate bid to return to former glories.
The acquisitions of Angel di Maria, Radamel Falcao and Ander Herrera have excited the club's supporters but a failure to significantly strengthen their defence has been regularly questioned.
And United's frailties were there for all to see at newly promoted Leicester on Sunday, as they tossed away a 3-1 lead and were eventually embarrassed in a 5-3 defeat.
United have won just one of their six matches this season and sit 12th in the Premier League table, with Neville – who made over 600 appearances and won eight titles for the club - seething at their latest inept performance.
The England assistant coach did not pull any punches, questioning their character on Sky Sports: "There's no doubt Manchester United are soft-centred. They're not tough enough.
"Going forward they're a lot better than they have been in the last 12-18 months and that's a positive.
"But in the first half, every ball that got played forward from distance, a Leicester player won it, whether it was the first ball or the second ball. In the second half, a decision goes against them but make no mistake about it they're soft-centred.
"I thought the midfield three for United and the back four got bullied."
Herrera put United 3-1 up in the 57th minute but David Nugent's 62nd-minute penalty changed the complexion of the game.
Esteban Cambiasso marked his home debut with a well-taken equaliser before further strikes to man-of-the-match Jamie Vardy and two-goal hero Leonardo Ulloa settled the contest.
And Neville felt several more defeats are on the horizon for Van Gaal's side this season.
"Credit to Leicester and Vardy and Ulloa - they really ran them ragged," he added.
"It wasn't intricate play or world-class football, it was just hard work.
"Grit, hard work, with a bit of quality mixed in with it and United couldn't deal with the physicality of that team.
"Their [United's] three goals were brilliant, they were absolutely fantastic goals but, they're going to get done week in, week out. That was a proper test for them today against Leicester."
