Manchester United boss Jose Mourinho believes it is becoming increasingly difficult for top clubs to purchase talent from their competitors.
Mourinho's Red Devils have won just six of their first 13 Premier League games, and seventh in the table.
As a result, the Portuguese manager is under pressure to improve his side, with many calling for Manchester United to enter the market in January to add to the squad.
However, Mourinho says that's a difficult prospect as he believes clubs are less willing to part with their top players as in years past.
The manager cited Tottenham as an example, referencing past deals for Michael Carrick and Dimitar Berbatov as benchmarks that are no longer possible in the modern day.
"It depends," he said. "I will give you an example, that I hope is a good example. I hope you can interpret it in the right way.
"Is Manchester United by history, by its dimensions, bigger than Tottenham? With all the respect - Tottenham is an amazing club - I think everybody would say yes.
"Can we go to Tottenham and buy their best players? No - because they don't sell or because they are so powerful that they can say no.
"A few years ago, who was Tottenham's best player? Michael Carrick. A few years later, who was the best player? Berbatov.
"Can we go now there and bring Harry Kane, Dele Alli, [Christian] Eriksen, [Heung-min] Son? Can we go there and bring the guys here? No.
"So, who is more powerful now. Them or us?"
As a result, Mourinho doesn't expect many massive deals to come in January, despite the club's needs.
Mourinho has repeatedly stated his need for another central defender, one of just several necessities he sees to improve his squad.
But, with clubs less willing to sell, he doesn't see it as a possibility in January.
"Football changes," he said. "It is more difficult to buy players of a high, high, high level.
"The clubs are more powerful and don't want to sell. Before, the smaller clubs, they were almost begging the big clubs, 'get my best player, I need to sell. Please, you are powerful, buy my best player'.
"At this moment, they don't want to sell. It is 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 and others cannot. Which is, I need a right back and a left back. Send away the ones you have and buy four at the same time.
"It is difficult, so we do what we can. Last season was fantastic to finish second and we are fighting hard. I know we have to do better than we are doing.
"But one thing is to do better and another thing is to try and compare ourselves with what Manchester United was in the past. Not just because Manchester United was fantastic, but because there was a big difference to the others and, at the moment, it is not like that."
Mourinho's Manchester United are back in action on Saturday against Southampton.