The Newcastle United midfielder was high on Ole Gunnar Solksjaer's transfer wishlist at Old Trafford this summer.
Longstaff impressed after breaking into Rafa Benitez’s first team in the second half of last season.
The 21-year-old – who is under contract at St James’s Park until 2022 after signing a new deal late last year – made 11 consecutive league and cup starts for Newcastle before suffering a season-ending knee injury in March.
Now fully fit, Longstaff has impressed Steve Bruce, Benitez’s successor at St James’s Park, over the past few weeks.
“We don’t want to sell our best young players, that’s for sure,” said the club’s head coach. “So, that won’t happen. I’m very confident (he will stay).”
The Gazette understands that Man United were told last week that Longstaff was not for sale – after they signalled their willingness to pay up to £35million for him. They are not expected to return with an improved offer before today’s 5pm transfer deadline.
Meanwhile, Longstaff’s younger brother Matty – who scored in last weekend’s 2-1 win over Saint-Etienne – is likely to stay at the club until January.
“With all young players, they want to go out and play for their development,” said Bruce. “However, I think playing with better players, playing with the first team every day, as he has done for the last two or three weeks, could see him improve.”
Check out our NUFC transfer deadline day blog here