"This season is going to be more difficult but I think we have better conditions to fight for the Premier League," Mourinho told the BBC.
"I think this season we are a little bit better equipped. We are against fantastic teams, against amazing investments.
"But I believe in our group, in our spirit, in our empathy, in our togetherness. I trust my boys and we are going to try."
Having lifted the Europa League in May, United secured direct entry to the Champions League group stage but the Portuguese manager believes they are still behind the continent's top clubs.
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"We went to the Europa League as one of the top teams, we go to the Champions League and we are not one of the top teams," Mourinho added.
"We have to be better, much better, for that objective. The base of everything is to find what I call a happy dressing room."
Mourinho has added striker Romelu Lukaku and centre back Victor Lindelof in the current transfer window but is still hoping to sign two more players ahead of the new season.
"One midfield player would give me more options. The other is an attacking player through the wings to give me more attacking options," he said.
The British media have linked United with Chelsea's Nemanja Matic and Tottenham Hotspur's Eric Dier as midfield options. Inter Milan winger Ivan Perisic is Mourinho's preferred choice to improve the attacking line, the reports have said.
United kick off their league campaign at home against West Ham United on Aug. 13.
(Reporting by Hardik Vyas in Bengaluru; editing by Sudipto Ganguly)
