Speaking after the Red Devils' 2-1 win over Leicester City in their opening Premier League match on Friday evening, the Portuguese was unable to hide his disappointment at missing out on his top transfer targets.
United moved quickly to add Diogo Dalot, Lee Grant and Fred to their ranks during the summer window, but Mourinho feels that his squad has not been significantly upgraded since the end of last season.
Having failed to land the new centre-back that he craved, Mourinho says that he is now in a difficult position.
"It's important to win and play well, we played well in periods against a good team that invest more than us," he told Sky Sports News after the match. "We must get used to teams who have players of the same quality that we have.
"Every team is a good team, forget the name, forget the history, forget the shirt. It will be a difficult season for everybody, not just for us. I had my plans for many months and I find myself with the market closed in a situation I did not think I would be in.
"It's the last time I speak about it. It's over, the market is closed. That's football management. I think football is changing and managers should be called head coaches. We have to wait until the end of November to understand if we can fight for the title."
Mourinho is now unbeaten in all 18 of his opening league matches of a season as a manager and has won nine out of 10 in the Premier League.