Rafa Benitez has shown a keen interest in reuniting with the Geordie midfielder having handed him his senior debut in December 2018.
It is understood the Toffees submitted a bid for the 24-year-old last week, however it was subsequently turned down by United.
Longstaff’s future at St James’ Park is up in the air at present, with his contract set to expire at the end of the campaign.
Howe, however, insists the player remains a key part of his plans, though admits any contract talks on hold during a hectic January transfer window.
That goes for Fabian Schar and Paul Dummett too as the latter prepares to return and help the club’s fight against Premier League relegation.
What Eddie Howe told NewcastleWorld about Sean Longstaff
"January is very, very intense for everyone connected with the club, Howe told NewcastleWorld.
“Trust me, I think this has been the most intense January that I’ve experienced.
"In terms of our current players that are out-of-contract, they may well have to wait towards the end of January (to discuss new deals).”
"Sean has a long-term future at this football club,” Howe insisted. “I’ve been impressed with him since I came in. He’s been committed, trained well and earned his place in the team.
"Against Manchester United, I thought he was excellent. The whole team wasn’t particularly good against Cambridge.
"We’re looking for a response from the team and Sean is part of that team."
What Eddie Howe said about Paul Dummett
Meanwhile, Howe is looking forward to welcoming Dummett back into the fold after recovering from a calf injury.
The 30-year-old was named as a substitute for the shock FA Cup third round exit to Cambridge United last Saturday - the first time he had been included in a matchday squad this season.
United exercised a one-year extension in Dummett’s contract last summer. He now has little under six months to earn a new deal at his boyhood club.
Howe said: “Paul Dummett is a massive part of our future, I really like him as a lad. He’s got a good temperament, really level-headed. His experience is going to be invaluable.
“I also like him as a player, although I haven’t seen a great deal of him due to injury but what I have seen, I really like.
“I’m very excited by the possibility of working with him.”