Two years on, and the midfielder is nowhere near a move to one of the Premier League’s elite clubs. He isn’t even a guaranteed starter for the Magpies.
Meanwhile, the Three Lions are set to compete in the final of Euro 2020 tonight – with Graeme Jones the only Newcastle representative.
As we begin preparations for the 2021/22 Premier League season, it is unclear what kind of role Longstaff will play when the campaign kicks off on August 15th.
But where has it all gone wrong for the oldest Longstaff brother?
The incredible rise of Sean Longstaff that nearly took him to Manchester United
The rise of Longstaff was sudden and explosive.
He spent the 2017/18 season playing in League One, joining Blackpool on loan and scoring eight goals.
The following season, he looked set for another loan spell, only for Rafa Benitez to stop him joining Portsmouth.
Longstaff then signed new four-year deal with Newcastle in November 2018, and made his Toon debut a month later against Liverpool.
This was the start of a whirlwind few months for the youngster.
Injuries handed him his first start on January 12th 2019, playing alongside Isaac Hayden in a 2-1 defeat to Chelsea.
This was the start of an eight-game run which would kickstart Longstaff’s career.
He would perform brilliantly alongside Hayden. The pair worked tirelessly in the middle, providing endless energy in the middle of Benitez’s 5-4-1 system.
Longstaff played a pivotal role in a famous win over Manchester City, winning a late penalty to allow Matt Ritchie to score the winner.
He then scored his first Premier League goal on February 26th, the second in a 2-0 win over Burnley.
Sadly, his next game against West Ham would be his last of the season. Longstaff picked up a knee injury after a challenge from Robert Snodgrass, ruling him out for the rest of the campaign.
Despite the injury, Longstaff looked set to become a star for Newcastle. He was primed to be a key player for years to come.
Then Manchester United came knocking.
Sky Sports reported in May 2019 that the Red Devils were interested in the English midfielder, as they looked to add domestic youth to their squad.
Then came the £50million valuation. That’s how much Newcastle United wanted for a player with just nine Premier League appearances.
Unsurprisingly, Manchester United did not meet the asking price.
Two years on, what does the future hold for Sean Longstaff at Newcastle?
It has been two years since Longstaff was valued at £50million. It would have made him Newcastle’s most expensive departure ever.
But he remains at St James’ Park, with a career that has somewhat slowed down since that remarkable rise out of nowhere.
Under Steve Bruce, Longstaff has struggled. He started the manager’s first game against Arsenal in 2019, playing in a midfield three alongside Jonjo Shelvey and Hayden.
We’ve seen flashes of brilliance from Longstaff during Bruce’s tenure, hinting at a return to his best. Sadly, they have only been flashes.
The 23-year-old has struggled since that knee injury.
And it remains unclear what his best role is and where he fits in in this Newcastle team.
When he debuted, it was all about his work rate and smart passing. It was nothing flash – he moved the ball quickly and efficiently.
But he’s not as good a defensive midfielder as Hayden. He’s not as good on the ball as Shelvey.
We’ve seen him play in a more advanced role, but he’s never going to get in the starting XI ahead of Miguel Almiron or Allan Saint-Maximin.
There is clearly talent there. We saw it during that remarkable run under Benitez.
But Longstaff has never really recaptured that form.
He enjoyed a decent run in the side late in the season after the Brighton debacle, and his energy helped move the club away from the relegation zone.
But he eventually lost his place in the starting XI.
Will the days of Sean Longstaff being a first-team regular ever return?
Two years on, and it’s crazy to think he was a target for Manchester United and valued at £50million.