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Jose Mourinho has warned that Manchester United face an 'absolutely difficult' transfer market as they attempt to credibly challenge Manchester City next season.

Mourinho is understood to want at least four new signings amid interest in Juventus left-back Alex Sandro,Tottenham centre half Toby Alderweireld,Shakhtar Donetsk midfielder Fred and Southampton defender Cedric Soares.

United signed three of their four priority targets last summer, having failed to recruit Ivan Perisic from Internazionale while Antoine Griezmann chose to stay at Atletico Madrid. United are resigned to Griezmann joining Barcelona this summer but reinforced their attack with Alexis Sanchez in January.

Mourinho has spent £286.2million in his two transfer windows in charge of United, who are 19 points behind champions City, and has cautioned it will be difficult to bridge the gap next term due to the nuances of the transfer market.

"The difference of points tell a lot," Mourinho said. "I always think that in the championships the truth is always there. In knockout competitions, sometimes in one bad day or in a lucky or unlucky moment, sometimes the truth is not always there. But in competitions, championships, 38 or 40 matches, the reality is always the reality of the numbers.

"And the reality of the numbers is between second and fifth the difference is three or four or five points, but the difference between the first and the second is a difference of 19 points and that's the reality of the numbers.

"Can we close that gap in one season with a very difficult market, absolutely difficult market? We are going to try."

City manager Pep Guardiola has indicated he could make just two summer additions, with Napoli midfielder Jorginho and Leicester forward Riyad Mahrez of interest. The transfer window opens on Thursday May 17.

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