The Portuguese coach has already invested more than £300m in squad improvements since taking charge, but he has hinted that he will spend conservatively when the transfer window reopens.
"We are not going to spend more than we can. We are not going to do anything crazy," Sky Sports News quotes Mourinho as saying. "We are just trying to improve a little bit more. That's what we are going to try.
"Everybody will try to improve - like we did from last season. Last season we finished sixth and this season we are going to finish, hopefully, second.
"Next season, we are going to try to improve, but let's see what happens. Maybe also City will invest massively and they don't let the others close the gap to them, I don't know.
"But it is not about us and them, it is about us and them and Liverpool and Arsenal and Chelsea and Spurs. It is not City and United. It is about the six."
United handed the Premier League title to City at the weekend when they lost 1-0 at home to West Bromwich Albion.