The Red Devils have been linked with a move to sign the Newcastle United midfielder throughout the summer transfer window.
Manchester United are in the market to sign young British talent this summer, as highlighted by their recruits so far.
The 20-time English champions are thought to have been quoted £50m for the 21-year-old despite his lack of experience.
Rafael Benitez’s dismissal has triggered speculation that Longstaff could move this summer.
However, his successor Bruce has ruled out selling Manchester United target Longstaff this summer.
“You never want to lose your best young players, and I think we’ve made it pretty evident that we won’t,” Bruce said, as quoted by Sky Sports.
“We’re concentrating on Sean now, getting his knee right. It’s his 10th or 11th training session since he hurt his knee.
“I’ll just repeat what the owner said – he’s not for sale.”
The Newcastle starlet has scored two goals in 11 games in all competitions since breaking into the Magpies team.
The No36 hasn’t represented England at youth level despite showing promise since breaking into Newcastle United side.
Longstaff is currently sidelined with a knee problem that has hampered his preparations ahead of the new Premier League season.
Manchester United signed Wales international Daniel James in an £18m deal from Championship side in June.
The Red Devils have also signed Crystal Palace defender Aaron Wan-Bissaka in a £50m deal to bolster their options at full-back.
The 20-time English champions have only managed to finish in the top four on two occasions in the past six seasons.
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