The Red Devils have endured a disappointing top-flight campaign in the Portuguese's first year at the helm, finishing sixth in the table regardless of their final-day result against Crystal Palace this weekend.
Mourinho believes that a "successful" transfer window is needed for United to bridge the gap on champions Chelsea, who currently hold a 24-point lead over his charges, but is happy with the progress being made.
"This season we are not close to winning the league. There are clubs with more stability than us, better squads, more time to work," Sky Sports News quotes him as saying. "I cannot say that but I'm pretty sure that from a mental and a tactical point of view, we have had a very good evolution.
"If, in the next transfer window, we manage to be successful, I think we can [challenge for the title]. The one that I really enjoy doing is the transition from the first to the second season because I know what I do, I know what I don't do. There is work to be done.
"The good thing is the club knows, the board knows, I know and we are together on this process. It was not the kind of club and team where you were ready to arrive and to go with everything and go direct to the big titles."
United face Ajax in the final of the Europa League next Wednesday, looking to add to their EFL Cup success from earlier this year.