United boss Jose Mourinho made signing a new centre-back one of his main priorities during the summer transfer window, but no deal materialised despite reports linking the club with Harry Maguire, Toby Alderweireld, Diego Godin and Yerry Mina.
The Red Devils' defeat at the Amex was littered with mistakes as Victor Lindelof and Eric Bailly struggled to cope with Brighton's attack, and Neville is unconvinced by Mourinho's options at the back.
"There's no doubt that before the first game of the season, Manchester United needed one or two authoritative new centre-backs. Jose Mourinho, in his own words, gave the list, and it wasn't delivered. For whatever reason - maybe they were too expensive, they didn't fit the club's philosophy or players weren't available," Neville told Sky Sports News.
"What we've seen today are two players in Eric Bailly and Victor Lindelof who are lacking confidence a little bit or maybe they aren't quite good enough. Maybe the answer lies somewhere in between. To me, they look like second-fiddle centre-backs. If they had, for instance, John Terry, Jaap Stam, Rio Ferdinand or Nemanja Vidic alongside them then they would probably be alright.
"You can get messed around a little bit and those two centre-backs today for Manchester United got messed around. It happens quite a bit. I'm not making excuses for them. There has been £120m of investment in that back four. You've probably got the best goalkeeper in the world, so from a back-five point of view, you could argue there has been serious investment.
"However, they are not good enough. Collectively they are not good enough and individually they are not good enough."
Only Arsenal, Fulham, West Ham United and Huddersfield Town have conceded more goals than United in their opening two games of the season.