The English ace was heavily linked with a £30 million ($37m) switch to Old Trafford while the transfer window was open, but a final deal never came to fruition.
Longstaff burst onto the Premier League scene last season, impressing in nine appearances for Newcastle before being struck down with a knee ligament injury in March.
He showed enough promise to catch Ole Gunnar Solskjaer's eye in Manchester, with the Norwegian boss favouring a transfer policy of targeting young, homegrown talent.
Longstaff ultimately ended up staying at St James' Park, but he found it difficult to concentrate on his pre-season duties amid swirling rumours over his future.
He has now revealed that Magpies teammate Matt Ritchie played a key role in helping him block out the ongoing speculation.
“You can sometimes get distracted by the outside noise and maybe for a little bit in the summer I was a bit,” Longstaff told Newcastle's official website.
“It’s hard not to whenever you go anywhere you see your face on the front of a newspaper and you turn the telly on people are talking about you. And it’s not like they’re talking about little things, they’re talking about things that can change your life really.
“The expectation is half what I’ve done and it’s probably a good thing. It does just go to show I did impress people when I got my chance last year. So for me, it’s about not listening to them and sticking to what I do. I speak to Matty Ritchie a lot, he’s someone I lean on in the squad. He tends to keep us grounded and he’s always there with good bits of advice.
“Sometimes you can get distracted by figures that are thrown out there and you’re doing things you wouldn’t normally try to do. I think I spoke to him about it and he said the reason people are speaking about you is because of what you did and keep everything so simple and ticking along and you’ve got the other stuff there if you need it."
The talented young star claims he has not yet returned to the same level he reached last season, despite earning a regular place in Steve Bruce's set up.
“I’m not back to where I was last year but I’m getting closer and closer and I think it’s just about working hard and getting back to that level," Longstaff said. "The outside noise is something I try not to listen to. It’s always going to be there now – there’ll be more eyes on me now.”
Ahead of Newcastle's trip to Anfield to face Liverpool on Saturday, Longstaff insisted he still needs to cut out moments of “sloppiness on the ball”, adding: "I’m only going to get back to that level with games and it’s going to take a bit of time. The kind of person I am, I want to be there straight away.
“The manager has given us that reassurance it’s going to take a bit of time. I know there’s things I can do better but it’s all learning and I’m still young. I’m still enjoying it, I’m still going to keep working hard on the pitch."