The Red Devils were previously seen as the standout club in English football and it was not uncommon for fellow title challengers to lose their best players to the lure of Old Trafford.
However, they have not bought a player from a top-four rival since signing Juan Mata in January 2014 when he was no longer in Mourinho's plans at Chelsea.
"Football has changed in the three years since Sir Alex Ferguson retired. This is the new football," Mourinho told Sky Sports News.
"Do you think I can go to Tottenham and buy two players to kill Tottenham? I can't. I cannot go to Arsenal and buy their two best players, or go to Chelsea and bring two of the players that I love very, very much. That time is over.
"The situation where you could start to become the champions by attacking your opponents is over in this country. If you get a player from a top club, you only get a player those clubs don't want to keep.
"You cannot attack your rivals any more. It used to be that when one of the big clubs was getting stronger, another of the big clubs was getting weaker. The problem we are finding is that we are getting stronger but so are the others. That means we have to grow up again and it is going to take more time."
United sit sixth in the Premier League table, four points outside the top four, ahead of their trip to English champions and relegation battlers Leicester City.