It appeared at one stage as though the Red Devils would be able to mount a challenge this term, but those dreams have quickly faded.
Arch-rivals City are cantering to the crown, leaving Mourinho’s men to scrap it out for a top-four finish and Champions League qualification.
Inconsistency has left United in this position, and Neville believes much more will be expected in 2018-19 as a man appointed to restore domestic dominance enters a potentially pivotal campaign.
Former United defender Neville told his podcast for Sky Sports: "The individual talent is there but to coordinate that into a team is Jose Mourinho's job in the next few months. He has to make them into a team.
"They look like a team that play five per cent below the intensity that they can play at and should play at. That's what Jose Mourinho has to do. He has to mould this talented group of players into a team that can come together and can win the title.
"Next season will be Jose's third season and you would think in his mind, and in everyone's minds, that he has to win it next season. They have to start preparing now.
"I still think they will finish second and they are still in the FA Cup and Champions League so there is a lot to play for, but they still have these moments where they let games drift. They miss chances, they are sloppy in the final third so many times and they have to improve that. They have to become killers in the final third and they are not at the moment. "
Neville added on the challenges facing the man at the Old Trafford helm, who recently committed to a new contract through to 2020: "The job will only be measured by whether Jose Mourinho wins the Premier League.
"Obviously winning the Champions League would be absolutely acceptable as well, but to win two trophies in his first season to now…he will be disappointed with how far Manchester United are behind Manchester City."
Having seen United suffer a shock 1-0 defeat at Newcastle in their last outing, City are now 16 points clear at the top of the table and could, if results go their way, wrap up the title in a derby date at the Etihad Stadium on April 7.