Liverpool spent £177m on new players for Jurgen Klopp's side over the summer, recruiting Naby Keita, Xherdan Shaqiri, Fabinho and Alisson.
The claim brought to mind Klopp's statement in 2016 when he said he would not spend like United following their world record purchase of Paul Pogba: "Other clubs can go out and spend more money and collect top players. I want to do it differently. I would even do it differently if I could spend that money."
Now Liverpool are in a similar position, Mourinho has had a mild jab back in their direction.
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Asked about his rivals for the coming season, Mourinho said: "I think Liverpool are trying to buy the title, but when I say buying I mean 'buying' with amazing investment.
"Chelsea has a fantastic squad, City have a fantastic squad too."
After a summer in which United failed to sign Toby Alderweireld, Mourinho praised Spurs for keeping their squad intact.
"And Tottenham made an incredible investment too by keeping their players," he continued.
"That's the best investment they could make because they could be having to spend £200m if they lose Harry Kane and Eriksen. The biggest investment was keeping their players. Arsenal will be better too, despite losing their first match. I think it's going to be a very hard season."
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