According to ESPN, United had hoped that the Serie A side might be willing to do business in January following their elimination from the Champions League at the group stages.
However, Napoli have reportedly held firm and United now believe that a deal is more likely to be struck at the end of the season than this month.
Staying in Italy, Inter are said to be offering Milan Skriniar improved terms amid interest from Old Trafford.
According to Tuttomercatoweb, United have already offered €60million for the Slovakia international, whose current contract runs until 2022.
The Nerazzurri are reluctant to sell Skriniar and would likely to demand a world-record transfer fee for a defender, perhaps approaching €90million.
Inter are also said to be prepared to let Ivan Perisic leave in order to finance a summer pursuit of Real Madrid's Luka Modric.
As reported by Tuttosport, Perisic was a long-term United target during Jose Mourinho's time at the club.
However, with Mourinho gone and Ole Gunnar Solksjaer happy with his options out wide, a move for the 29-year-old appears unlikely.
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