The Old Trafford side were distant runners-up to Manchester City in the Premier League last term and have not won the title since legendary manager Sir Alex Ferguson departed in 2013.
The Portuguese has played down their chances of turning the tables on Pep Guardiola's side over the next 10 months and claims it will be a significant achievement even to secure a place in Europe’s top club competition.
"We are going to try, but it's a very difficult job in front of us because second was really hard and this season is going to be really hard for everyone to finish top four,” he told Sky Sports when asked about the Red Devils’ prospects of finishing first.
Mourinho has pinpointed the usual suspects as United’s chief threats but claims that Tottenham, who became the first Premier League side ever to not make a single summer signing since the introduction of the transfer periods, had the most successful window.
"Liverpool are trying to win the title with their amazing investment. Chelsea has a fantastic squad, Manchester City has a fantastic squad too, Tottenham made an incredible investment by keeping their players, that's the best investment you can make,” he explained.
“They could be spending £200m but if they lose Harry Kane or Christian Eriksen… the biggest investment was keeping all their best players. Arsenal will be better even though they lost their first match. I think it's going to be a very hard season."
United opened their Premier League campaign with a 2-1 home success over Leicester, in which Paul Pogba and Luke Shaw netted, and on Sunday face Brighton away from home.