The Red Devils are expected to want to pay a maximum £25m for the 21-year-old with boss Ole Gunnar Solskjaer vowing to walk away from over-priced targets.
In a summer where Rafa Benitez and Ayoze Perez have already left Newcastle and the club is even more of a shambles, few would blame Longstaff if he wanted to get out.
The central midfielder started in eight games in the Premier League, netted his first goal for the first team vs Burnley and provided a crucial assist vs champions Manchester City.
Nevertheless, though, despite having made just 12 appearances this season, the Magpies' starlet seemingly did enough to pique the interest of one of the Premier League's heavyweights.
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Leicester are demanding £90m for the England centre-back but United are hoping to strike a deal around the £75m figure that Liverpool paid Southampton for Virgil van Dijk previous year.
It is added, though, that Longstaff nevertheless remains "fully committed" to Newcastle.
Although United recognise they are operating in an inflated market and there is a premium on young British players, there is some concern that there is nearly an additional tax on players they are interested in signing.
However, the same article says that the player himself is still hopeful that a deal can be agreed this summer.