Longstaff, who has eight Premier League starts to his name, could become the club's third signing of the transfer window.
The 21-year-old showcased the kind of traits rarely-possessed by a player of such tender years, with his increasingly-prominent role in Newcastle's midfield ranks having only been cut short by a season-ending knee injury.
Sean Longstaff spent loan periods at English League One club Blackpool and Scottish Premiership side Kilmarnock.
According to widespread reports, a £50m price tag has been put on the 21-year-old's head.
Manchester United manager Ole Gunnar Solskjaer has turned his focus towards recruiting young, homegrown talents with potential this summer, as seen by the signings of Daniel James and Aaron Wan-Bissaka.
Longstaff ended the 2018-19 season with just 13 appearances to his name, in which he found the back of the net on two occasions, while setting up one goal as well.
The outfit in question come in the form of Manchester United.
Manchester United have also snapped up England Under-21 worldwide Aaron Wan-Bissaka in a £50m deal from Premier League rivals Crystal Palace. However, the Mancunians are unwilling to offer more than £20 million for Longstaff and could now pull out of the deal, given the Magpies' exorbitant demands.
Leicester are demanding £90m for the England centre-back but United are hoping to strike a deal around the £75m figure that Liverpool paid Southampton for Virgil van Dijk previous year.