The Red Devils boss signed four players on arrival at Old Trafford last summer, all from abroad.
Zlatan Ibrahimovic was brought in on a free from Paris Saint-Germain, Henrikh Mkhitaryan from Borussia Dortmund, Eric Bailly from Villarreal and Paul Pogba from Juventus.
However, when Jose Mourinho was at the helm at Chelsea a decade ago, he was able to swoop from rivals in England’s top flight.
The growing funds for clubs through TV revenue though has seen competition for the title strengthen.
Jose Mourinho admits it is now difficult to sign from Manchester United's Premier League rivals
Manchester United, Manchester City, Liverpool, Arsenal, Chelsea and Tottenham can all now be considered real title contenders each season.
And Mourinho admits that makes it difficult to price teams out of keeping their star assets.
“Here, everyone is powerful now,” Mourinho told France Football.
“The scenery has changed. If I want to buy a player from Tottenham, I can’t.
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Who could Jose Mourinho target in the summer?
Manchester United boss Jose Mourinho looks to sign players from abroad now
Nor can I buy a player from Manchester City or Arsenal. That was not the case before
“Nor can I buy a player from Manchester City or Arsenal. That was not the case before.
“At Chelsea, I brought in Shaun Wright-Phillips, who was the best player at Manchester City or Ashley Cole from Arsenal.
“Sir Alex, when he wanted the best player from Tottenham, he bought Michael Carrick and then [Dimitar] Berbatov.”
Chelsea star Willian is said to be one of those in the Premier League on Jose Mourinho's radar
United have been linked with a host of players from their Premier League rivals ahead of the summer transfer window.
While the Red Devils are pushing for a return to Champions League football, they are also trying to attract new talent to Old Trafford.
Arsenal’s Alex Oxlade-Chamberlain, Spurs’ Toby Alderweireld and Chelsea’s Willian are all reportedly on Mourinho’s radar, although no deals have been agreed yet.