The Red Devils ended 2017-18 as the best of the rest in the English top flight, accruing 81 points in total to finish behind only runaway champions Manchester City.
United boss Mourinho believes that each of his side's top-six rivals are stronger this time around, however, and concedes that only by finishing top will this season be considered an improvement on last term.
"Last season we put ourselves in a difficult position, because to do better we can only finish first," he told Sky Sports News. "When I read sometimes and watch sometimes it looks like we finished sixth or seventh and everybody did better than us. We finished second.
"Liverpool are trying to win the title with their amazing investment. Chelsea have a fantastic squad, Manchester City have a fantastic squad too, Tottenham made an incredible investment by keeping their players, that's the best investment you can make.
"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."