United have spent almost £400 million ($510 million) on players since Mourinho arrived at the club in 2016, but the manager claims buying top-level players is becoming increasingly difficult.
Mourinho claims the balance of power has shifted in the Premier League – using the example of Tottenham, who spent no money on players during the last transfer window, but also managed to keep their key names, in Harry Kane, Dele Alli, Christian Eriksen and Son Heung-Min.
There was no attempt by United to sign any of those four players, but Mourinho is adamant Tottenham would have refused to sell even if there had been.
He argued it did not used to be the case, with United prising England midfielder Michael Carrick from Tottenham in 2006, then luring Bulgaria striker Dimitar Berbatov from White Hart Lane in 2008.
"Is Manchester United by its history, by its dimension, bigger than Tottenham? With all respect, Tottenham is an amazing club, but I think everybody would say yes," Mourinho said.
"Can you buy Tottenham's best players? No, because they don't sell. Of course, they are so powerful that they can say no.
"A few years ago, who was Tottenham's best player? Michael Carrick. And a few years later, who was the best player? Berbatov.
"Can we go there now and bring Harry Kane? Dele Alli, Eriksen, Son? Can we go there and bring those players here? No. So who is more powerful now? Them or us?"
In what appeared to be a covert criticism of Manchester City, Mourinho suggested financial restrictions sometimes did not apply in the Premier League.
Mourinho said some clubs could buy multiple full-backs at the same time, a comment he has made before when referencing City's transfer policy.
City manager Pep Guardiola bought three-full backs in 2017 while allowing another four to leave as he reshaped his squad.
"It's more difficult to make the team better and stronger with buying –- unless you do what some clubs do, and for some reason they can do this and others cannot, which is say: I need a right-back and a left-back, send away the ones you have and buy four at the same time," Mourinho said.
After making this comment, Mourinho was asked why United did not have the financial muscle required to buy the best players.
"I cannot say the answer because one of the answers you had a few weeks ago. I don't want to speak about that. But one of the answers you had a few weeks ago," he said.
He refused to confirm whether this was a reference to allegations made at the start of November that Manchester City had circumvented UEFA's Financial Fair Play regulations.
"We do what we can. Last season, it was fantastic to finish second and we are fighting hard," he added.
"I know that we have to do better than what we are doing, absolutely, we have to do better, but one thing is to do better and another is to compare ourselves with what Manchester United was in the past because it is impossible.
"That's not just because Manchester United was fantastic, but because there was a big difference to the others and now it is not like that."
Asked if he would buy any players in January, the manager said: "Honestly, I don't think we are going to sign anyone. If we do, I would say one player. I don't see us going more than that."AFP