Mourinho was left frustrated by a lack of transfer activity before the Premier League deadline after only Fred and Diogo Dalot arrived, and his concerns were laid bare as United crashed to a 3-2 defeat.
A new centre-back was Mourinho's top priority amid links with Tottenham star Toby Alderweireld and Leicester City's Harry Maguire but neither were signed, and United's partnership of Eric Bailly and Victor Lindelof were exposed at The Amex as defensive errors contributied to the Red Devils' first loss of the season.
Dissecting United's result on the road, former captain Neville said Ed Woodward and the club's hierarchy must back Mourinho financially if they are to make up for the mistakes of the past following Alex Ferguson's departure in 2013.
"He is in his third season and I think he will feel extremely disappointed that he wasn't supported in the transfer market," Neville said on Sky Sports. "You can't leave a house three-quarters built. You have to finish it and complete the build. Then you can think about selling it or renting it because nobody is going to pick it up off you half-built.
"Manchester United have had a problem since Sir Alex Ferguson retired in the transfer market with David Moyes and then Louis van Gaal.
"You could argue £350 million to £400m of the £700m that has been spent has not been wasted, but it's not value for money.
"He needs further investment. It's obvious that he does and the club should support him in that because you can't withdraw part way through a process. They have to go for it.
"It will get better and Manchester United will get stronger and won't be easy to play against. I do think they will finish in the top four, but the top four historically has never been the benchmark of what Manchester United are. They have always judged themselves by Premier League titles.
"At this moment in time, that looks unlikely. Obviously it is still early in the season, but it does look unlikely that Manchester United are anywhere near good enough, as we sit here today.
"Forget everything else, that back four is not good enough to win a Premier League title. Let's start and finish with that.
"The goalkeeper is the best in the world, the midfield is strong with Paul Pogba, Nemanja Matic and Fred and up front there is Romelu Lukaku, Anthony Martial, Alexis Sanchez, Marcus Rashford, Jesse Lingard, Juan Mata and a lot of good players. But that back four looks like it will always cause problems and always concede goals. It doesn't have authority and that is a problem."