The Portuguese has spent roughly £300m on new recruits since taking over from Louis van Gaal in the summer of 2016, including £89m on Paul Pogba and a further £75m - potentially rising to £90m - on striker Romelu Lukaku.
Man City have brought in 16 new players during the same timeframe, meanwhile, and fellow European heavyweights Paris Saint-Germain smashed the transfer record earlier this year by recruiting Neymar from Barcelona for £198m and reaching an agreement to bring in Kylian Mbappe for a slightly lesser fee.
Mourinho claims that it is becoming tough to keep up, even if he has had ample backing himself, and does not see the situation changing across the next couple of windows.
"It is difficult [to compete] but it is possible. Sometimes if you don't have that financial profile of club where there are no limits and the only thing that matters is to get the best, there is only one way which is patience, calm and time," he told reporters. "I think next season we will get two or three more players. But we will lose two or three.
"[We are] not going to have a dramatic change or a dramatic improvement. When [Jurgen] Klopp arrived in Liverpool he trusted his quality as a good coach, which he is, but I think now he realises that is not enough to be a champion, to win the Champions League and to win trophies.
"So now you can see Liverpool with Naby Keita, who they have already bought for next summer, and with Virgil van Dijk, they realise they have to go to the same levels of the crazy market. Because if they offer £40m, Southampton don't sell them the player. Some clubs invest with limits, some go absolutely strong in the market.
"What Paris Saint-Germain did this summer with Neymar and Mbappe, they get two of probably the four best attacking players in the world. Two at the same time. And then players like [Angel] Di Maria, [Julian] Draxler, [Javier] Pastore, Lucas Moura - they are second choices. So, money makes a difference."
Mourinho insisted earlier this week that the £300m he has spent since arriving at Man United is not enough to turn the club into a superpower.