Despite spending £455 million on transfer fees in the last three years, United currently sit 24 points behind Chelsea heading into the final round of matches.
And Mourinho claims the financial clout of other clubs is the reason his club have struggled to compete in recent seasons.
"I always had the question mark, what happened that stopped Man United being successful in the last three, four, five years?" he said.
"One answer: the reality of the Premier League, the economic power in many clubs. The stability that clubs manage to give.
"There is work to be done on the Manchester squad. The good thing is the club knows, the board knows and we are together on this process."