Longstaff has just months remaining on his current deal with Newcastle, and no deal has been agreed yet.
Dummett says that he has advised Longstaff to not be influenced by what others are saying, when it comes to deciding his future.
But the Welsh international believes that Longstaff wants to stay at St. James’ Park.
“We spoke a little bit in the changing rooms about certain things and I just said you know everyone knows that he's out of contract at the end of the season so he's got to make his own decision on what he wants to do,” he said.
"Obviously he's a Newcastle fan. I'm sure he'll want to stay in Newcastle for many years and play a lot of games, the same as what his brother will want to do."
Longstaff’s future at Newcastle has come into sharp focus in recent times, as he impressed so much in Steve Bruce’s side’s last match.
The youngster made his senior debut for Newcastle and scored the winner as the Magpies beat United.
Longstaff now has staked his claim for a spot in Newcastle’s regular line-up, despite only being 19, but his goal is also sure to have put other clubs on red alert over his contractual situation.
Newcastle’s next game comes against Chelsea after the international break, and it remains to be seen whether Longstaff will keep his spot.
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