The academy graduate has progressed into an important member of Eddie Howe's squad, contributing 16 goals and 10 assists from 182 appearances for Newcastle in total.
Thirty-five of those came in the Premier League last season, yet the Magpies' complex position with the division's Profit and Sustainability regulations makes anything possible at St James' Park.
According to Football Insider, the North-East outfit would be willing to sell the 26-year-old for a fee of £20m over the coming months.
This stance has seemingly been taken due to Longstaff having just 12 months remaining on his contract, Newcastle not prepared to risk losing a key man on a free transfer.
While it remains possible that fresh terms could be penned in due course, it appears that there are no signs of that occurring in the short term.
Man United to lose youngster on free transfer
Meanwhile, Manchester United will reportedly suffer another blow when it comes to losing one of their prospects on a free transfer.
Omari Forson has already taken the decision to search for first-team football elsewhere having remained on the periphery of Erik ten Hag's squad during the second half of the season.
As per Manchester World, Shola Shoretire will soon follow suit and depart Old Trafford.
There were once high hopes of the winger becoming a permanent member of the senior ranks when breaking into the team prior to Ten Hag's arrival.
However, the 20-year-old has not been given any opportunities by the Dutchman in two years, being restricted to Premier League 2 and EFL Trophy outings during 2023-24.
As a result of not being rewarded for chipping in with nine goals and four assists from 12 appearances, Shoretire has instead rejected the chance to extend his contract.
Former Barcelona man attracting interest
Over in Spain, there is reportedly plenty of interest in former Barcelona forward Martin Braithwaite, who is contracted to Espanyol.
The 33-year-old has just scored 22 goals in the second tier of Spanish football, helping his club earn a return to La Liga.
However, according to transfer expert Fabrizio Romano, there is a clause in the Denmark international's contract that would allow him to depart on a free transfer before July 15.
That has inevitably led to clubs from Europe showing an interest, while it is alleged that Saudi Pro League clubs are also keen.
If Braithwaite does not depart the Stage Front Stadium by the aforementioned date, a new release clause of €15m (£12.68m) will be inserted into his contract.