Mourinho's United head into their trip to West Ham on Thursday 20 points adrift of runaway neighbours City in the Premier League.
The second-placed Red Devils – who defeated all of the Premier League's top six teams this term – crashed to a seventh defeat of the season at Brighton, who claimed a shock 1-0 win on Friday.
Mourinho was scathing of his players afterwards, having presented the likes of Anthony Martial and Marcus Rashford with opportunities to stake their claims for starting berths in the absence of Romelu Lukaku and Alexis Sanchez ahead of the FA Cup final against Chelsea.
The Portuguese boss said United must improve their mentality, while investing in new signings to bridge the gap in the Premier League and Champions League next term.
"How many points separate us from Manchester City? Not two, three or four - a lot of points," Mourinho told reporters.
"I think we need both things that you mentioned [more quality or mentality to bridge the gap in the Premier League]. Some of the people we have, they have to be more consistent in their performance, and we need more quality.
"I trust some [players] more than others that can bridge that gap. That's normal. I know the players. I worked with the players for a couple of years. I know them better than anyone.
"And of course, some of the players I am totally convinced are going to make it. Some others I have good hopes that they are going to make it and some others I don't believe they are going to make it."
Mourinho added: "Of course we can keep [those players]. Unless there is a perfect squad, where you buy one goalkeeper you don't like, you buy another one; you buy a central defender you don't like, you buy another one. Unless you can do that, and you go in the direction of the perfect squad, you have to keep players you don't love.
"When I say love, I say it footballistically - players you don't totally believe have the level of the squad but you have to keep. If you go to the Premier League managers, I don't say every one, but I say the majority of them will tell you the same. In my squad, I have some players where I would like to have better than what I have. That's basic."