United dropped more points in an increasingly one-sided title race courtesy of a 2-2 draw at home to Burnley this afternoon, meaning that they could go into the final game of 2017 sitting 15 points adrift of their local rivals.
Mourinho has already spent close to £300m on new signings since taking over at Old Trafford, but he claims that City still have a major advantage in the transfer market.
"One thing is a big club and another thing is a big football team. They are two different things," Mourinho told reporters.
"We are in the second year of trying to rebuild a football team that is not one of the best teams in the world. Manchester City buy full-backs for the price of strikers. When you speak about big football clubs, you are speaking about the history of the club."
When it was pointed out that United have spent roughly £286m under Mourinho, the Portuguese added: "OK, [but] it is not enough. The price for the big clubs is different than for the other clubs. The big historical clubs, they are normally punished in the market because of their history."
United are now winless in their last three matches across all competitions heading into Saturday's showdown with Southampton at Old Trafford.