Manchester United are already damaging their chances of winning next season's Premier League title, according to Gary Neville.
This season's challenge is all but over after a defeat at Newcastle United allowed Manchester City to open up a 16-point lead at the top of the table with just 11 games left.
Mourinho has already conceded that the trophy is destined for the Etihad Stadium.
But the United manager has also suggested he will use the rest of the campaign to build towards next season's title tilt -- something Neville said was damaged with the 1-0 defeat at St. James' Park on Sunday.
"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," Neville told Sky Sports. "They have to start preparing now, and days like today do damage confidence and take away the work you are trying to do.
"They still have these moments where they let games drift. They missed 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."
After winning five consecutive games to start 2018, United have lost two of their last three in the Premier League.
They will aim to bounce back on Saturday when they visit Huddersfield Town -- where they lost 2-1 in October -- in the fifth round of the FA Cup.
And, according to Neville, Mourinho's first job is to blend a group of good players into a team.
"The individual talent is there but to coordinate that into a team is Jose Mourinho's job in the next few months," Neville said. "He has to make them into a team.
"They look like a team that play five percent 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."