Manchester United linked with shock move for Newcastle United midfielder
Longstaff broke into Rafa Benitez's side in the second half of the season after a successful loan at Blackpool last term.
The 21-year-old was handed a chance after manager Rafa Benitez lost Jonjo Shelvey and Mohamed Diame to injuries and Ki Sung-yueng to the Asian Cup.
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Hide AdLongstaff made eight successive Premier League starts and scored two goals for the club before suffering a knee injury in early March.
The Newcastle-born player is working through the summer in an attempt to be fit for the start of pre-season in early July.
Sky Sports claim that Man United are "looking" at Longstaff, who signed a new four-year contract in December.
Newcastle, however, have made it clear that Longstaff is "not for sale".