The Red Devils boss Ole Gunnar Solskjaer expects to build on the £15m capture of Daniel James from Swansea by securing the deal.
Earlier this month, the midfielder promised to remain in Newcastle in an interview, the United are confident they can land the 21-year-old.
The uncertainty at Newcastle, with a proposed takeover imminent and the future of boss Rafa Benitez, is expected to work in Solskjaer’s present.
Longstaff, who made 11 successive starts for Rafa Benitez’s side last season, before he suffered knee ligaments against West Ham on March 2, which bring an end to his season.
Solskjaer had seen enough of the player, despite that setback, he convinced that the player is fit for the profile of the kind of player he wants at Old Trafford.