United recorded their best finish since Sir Alex Ferguson's retirement last season, but they were still 19 points adrift of champions Manchester City and endured a frustrating transfer window due to their failure to bring in defensive reinforcements.
City have once again started the new season brightly, whereas United fell to a shock loss at the Amex in just their second game, and Neville believes that his former club will once again have to target a top-four finish first and foremost.
"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," Neville told Sky Sports News.
"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.
"Jose Mourinho knows about Premier League title-winning teams and about winning titles in different countries. The base and foundation of that is a good defence."
United's next match sees them host Tottenham Hotspur at Old Trafford next Monday night.