Starsport understands Ole Gunnar Solskjaer wanted to sign the 21-year-old prior to the Red Devils’ tour of Australia.
A deal was not sealed for the Geordie midfielder, though, as Newcastle raised their valuation to £50m.
It was felt at the end of June that Longstaff could be snapped up for closer to £25m, but United have baulked at the hefty price-tag.
The Magpies academy product only burst onto the first-team scene last season when Rafa Benitez took a chance on him at Christmas.
A spell in the side saw Longstaff earn rave reviews for his performances as he helped Newcastle earn some memorable victories, including against champions Manchester City.
A knee injury then cut his season short but Manchester United saw enough to make their stance clear, with Longstaff having confirmed he is close to returning as he pushes to get fully fit for the start of the campaign.
Benitez’s exit hurt Longstaff too, with Steve Bruce having come in as an underwhelming replacement.
Newcastle fans are already fearing a relegation battle this term, but Longstaff potentially staying put would be a big boost.
READ MORE: Newcastle and Man Utd both feature in our Transfer News LIVE blog
Regarding United’s pursuit, Longstaff told the Chronicle from China: “I just keep my head down.
“I am just trying to get fit – that’s all I can focus on. I am still a Newcastle United player.
“I want to get back on the pitch as soon as I can. That is what I am looking forward to doing.
“I make more and more progress everyday, that’s all I look forward to. I can’t wait to get on the pitch and impress the new boss.”
His rise has been meteoric to say the least from just eight Premier League appearances, and Longstaff added: ”If somebody had said to be me a year ago I would be in this position, I would probably have laughed at them.
“It is flattering but my job is at Newcastle”
“This time last year I was deciding whether to go to League One (Portsmouth) on loan. A year on, you are seeing your name linked with one of the biggest clubs in the world.
“For me, it is all a positive really. It gives you even more confidence. When I was in for a short amount of time people took notice of me. It is flattering but my job is at Newcastle.
“The new boss is coming in and it’s a clean slate for everybody – including me.
“I just get my head down and work on the pitch. Like anybody knows you are no use to anybody if you aren’t fit.”
On Benitez’s departure and Bruce’s arrival, he continued: “Nobody wanted to see Rafa Benitez go.
“But that’s what football is – it changes, all of the time.
“I would say everybody was sad to see Rafa go but the squad is excited now as a new manager is coming in.
“Rafa leaving is in the past. It has to be positive now from within and we must all look forward.”