#1 Team spirit

There was an air of negativity around Old Trafford because of Mourinho's less than enthusiastic comments regarding the signing of players before the end of the transfer window, but those signs were not on display as the whole team celebrated the victory together.
Leicester had a really successful transfer window, and thus they have a team capable of finishing in the top six, but United's professional performance against them was possible only due to the amazing team spirit.
It may have appeared pessimistic from the outside, but players like Lukaku openly backed the manager and reiterated that the mood inside the camp was very friendly when it came to the relationship with the manager, and the players answered the questions on the pitch.
Mourinho's own reaction to Shaw's goal meant that all is well, and the manager as well as his players are ready for the title challenge this season.