The Red Devils opened their 2018-19 campaign with a 2-1 victory over Leicester, with the likes of Paul Pogba impressing in a narrow win.
United’s stand-in skipper has, however, faced criticism along with a number of team-mates after suffering a 3-2 defeat at Brighton last time out.
Mourinho admits that his side have let themselves down at times, with it vital that the collective level of performance is improved ahead of a home date with Tottenham on Monday.
He told MUTV: "You know, just looking to Leicester and Brighton. Against Leicester we didn't make mistakes, we played well, we won. Against Brighton, we made mistakes, we paid for the mistakes and we lost.
"So, I think you look at football in a very pragmatic way, normally you get what you deserve, so if we want to win against a good team against Tottenham, we cannot make mistakes."
Mourinho believes his United squad are ready to make amends for their Brighton setback against Spurs.
He has been impressed by the effort he has seen in training, with everyone connected with the club looking for a welcome morale-boost at Old Trafford.
The Portuguese added: "I think nobody is happy, obviously.
"But I have enough experience to know that in football you cannot look back for good results, for bad results and you have always to look to the next one. That's our life.
"So, I'm really happy with the work we did in the week, with the players' commitment and I look forward for the game.
"You can look at [having to wait until Monday] from two different perspectives. By one side I would say yes [it is frustrating] but, on another side, I would say we need time to work and some time to try to recover some players from injury, so it was not bad for us."