City are currently top of the Premier League, 15 points clear of second-placed United, and are yet to lose in domestic football.
Pep Guardiola oversaw a major influx of talent in the summer, spending £221.5m to sign defenders Benjamin Mendy, Kyle Walker and Danilo, goalkeeper Ederson, midfielder Bernardo Silva and young prospect Douglas Luiz.
United, meanwhile, spent just over £145m, but Mourinho has again highlighted the perceived limitations of his side compared to the league leaders.
"You think the club can put here now £600 million and let's buy six players of £100 million? The club cannot do that," he said in an interview with United's official website .
"I cannot expect the club to do that, so it's not critical and you can see how the market is, especially with the top teams. But, you know, without taking any credit away from Manchester City and Pep [Guardiola] and his staff and the players, they obviously have lots of credit in what they are doing. But Pep arrives, he has the goalkeeper of England [Joe Hart], he doesn't like him so he buys the goalkeeper of Barcelona [Claudio Bravo], he doesn't like him so he buys another one [Ederson]. Now he likes.
"He has [Pablo] Zabaleta and [Aleksandar] Kolarov - two very good players but more than 30 years old. He wants to replace, he doesn't replace with two, he replaces with three. One from Tottenham [Kyle Walker], one from Monaco [Benjamin Mendy] and one from Real Madrid [Danilo] as an example.
"Can we buy six or seven players at the same time? Can we invest £600-700 million? No. So, it's difficult. I think the last years, the market is going in such a direction or you belong to one of these clubs where there is no limit and you just buy what you want and there is no limit, there is no financial fair play, there is nothing, you do what you want - or it's hard."