United have struggled to capture top targets at times in the last few years but missing out on Longstaff, whose contract expires in 2022, would be a damning indictment of their standing in the transfer market.
“If they can't sign [Youri] Tielemans and Longstaff then we're done,” one supporter said in reference to the news. “They are squad players that we don't have any competition for. And we really need them.”
Newcastle said “the process to appoint a successor will now begin” which could leave the players without a manager when they return to training on July 4.
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Rafa Benitez: 'Devastated' Newcastle fans react to departure news
Sean Longstaff established himself as a regular starter at Newcastle before suffering a knee injury (Image: Getty)
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That prompted one member of the Old Trafford faithful to post: “Longstaff to mufc [thumbs up emoji].”
“Best time to buy longstaff from newcastle for cheap. OLE WAITING FOR WHAT?” asked another.
There are still some Longstaff sceptics, though, with one United follower writing: “So does this mean that Sean Longstaff will be Pogba's replacement [unimpressed face emoji].”
Another plainly said: “Benitez was my only hope to stop Longstaff to United.”