Jose Mourinho has warned the level of spending on players is reaching a "dangerous" level, but defended Manchester United's own big-money transfers.
The Red Devils reportedly forked out more than £120million to sign Romelu Lukaku and Victor Lindelof this window, while United smashed the transfer record to sign Paul Pogba from Juventus last August.
Mourinho says it is normal the world's top talent should demand such excessive fees, highlighting Paris Saint-Germain's pursuit of Barcelona star Neymar for £196m as an example.
But the Portuguese believes it is a problem that clubs are willing to sign off on huge transfers that do not reflect the true value of players.
"I always thought the problem is not what you pay for these kind of [top] players, I don't think the problem is what you pay for Pogba, I don't think the problem is going to pay crazy for Neymar," Mourinho told reporters in Washington DC where United are preparing for to face Barcelona in their pre-season tour of the United States.
"I think the problem is with the other group which is a big group because players like Pogba, there is one or two [big] transfers [like that] per transfer window. The other ones are where you have 100 transfers and for me that is the dangerous area of the market.
"Some clubs are paying or they don't buy because they don't accept the numbers that are now ruling the market, or to do it they have to go the same levels and for me that's what worries me a little bit because now we speak about £30 million, £40 million, £50 million in such an easy way.
"I don't think it will ever kill the big clubs because the big clubs will always have the potential and conditions to produce this kind of money and not to be in problems.
"I think [for] some other clubs, there will be problems when they realise that the money that is coming in and is going out and I also think that the financial fair play authorities, they have big work to do.
"Big work to do because probably there are some strategies of disguise but I have to believe that the financial fair play is going to have difficult work to do."
While United shelled out on the move for Lukaku, they have thus far refused to yield to the apparent £50m and £45m valuations of Tottenham's Eric Dier and Inter's Ivan Perisic.
And Mourinho believes the best approach taken in the current window has come from Spurs, who have yet to make a signing and received a reported £50m from Manchester City for Kyle Walker.
"Everybody speaks about the dimension of the investment at Man City, but there is another team that I feel the dimension of their investment is also phenomenal – Tottenham," he added.
"I think until now they spent zero pounds, right? For me the dimension of their investment is amazing. They keep everybody they want to keep.
"They keep Dele Alli, Harry Kane and Toby Alderweireld, they keep Eric Dier, they keep everyone they want to keep."
