Longstaff was recalled to the starting XI for Saturday’s game against Tottenham Hotspur, which the club won 1-0.
And the midfielder – who started the previous weekend’s 3-1 defeat to Norwich City on the bench – played his part as the club secured an unexpected first Premier League win over Steve Bruce.
“I think no-one expected us to get anything,” said Longstaff, who played ahead of Jonjo Shelvey. “I think we came with the mindset of the performance having to be better than last week. That was the bare minimum. Obviously, that’s turned into a massive result for us. It’s a brilliant three points.”
Joelinton gave United a first-half lead at the Tottenham Hotspur Stadium, where Newcastle were backed by 3,000 fans despite the closure of Kings Cross train station for the weekend.
Newcastle produced a backs-to-the-wall performance to keep Mauricio Pochettino’s side at bay after the break.
“It was unbelievable – I think it shows the spirit of the team,” said Longstaff. “Obviously, I think there’s been a lot of criticism of the team in the last week. We’re not stupid, we deserved it last week. This week, I thought we were outstanding, especially in the second half. People were putting their bodies on the line.
“You saw the reaction that got from the crowd, which was unbelievable. The travelling fans were immense once again. You can see what it meant to them and to us at the final whistle.”
Bruce’s tactics were criticised after the Norwich defeat. United’s head coach – who rounded on some of his critics ahead of the Tottenham game – switched to a 4-5-1 system in the capital.
“At the end of the day we’re professional footballers,” said Longstaff. “A lot of it comes down to how you play on the day. Obviously, we had a slight little change, and it probably did work a little bit better.
“That was probably because the performance was a lot better. It’s something to build on. We’re happy in the changing room, happy for the manager to get his first win. It’s the start, hopefully, of a positive run.”
Longstaff had watched from the bench in frustration at Carrow Road as Teemu Pukki scored his first two goals for Norwich. He came off the bench and set up Shelvey's late consolation strike.
Bruce said after the game that he had been “protecting” the 21-year-old – who suffered a season-ending knee injury in March after breaking into Rafa Benitez's side – as he recovers his form and fitness.
"He's been out for the best part of five months, and the one thing I needed to do was just try to protect him a little bit and have a conversation with him to say, 'look, it's normal that sometimes you might have to miss a day or two', just to make sure he was ready," said Bruce.
Longstaff, impatient to get back to his best, had wanted to play from the start, but he understood Bruce's reasoning.
“You want to play as many games as you can,” said Longstaff, who could start Wednesday night’s home Carabao Cup tie against Leicester City.
“Arsenal (on the opening weekend of the season) was my first proper game in five months. You can play all the pre-season games you want, but there’s nothing quite like playing in the Premier League.
“I’m a professional. You get your head down, work hard and do as much as you can to get back in the team. You cheer on the lads when you’re on the sideline, and this week I got my chance again.
“There are probably little bits of quality I need to touch up on, but I think that’ll only come with playing games. I’m playing better week on week.
“Like the manager said, it’s probably going to take a bit of time. The type of person I am, I want to be back at that level straight away. There’s enough good people around me to not let me get too angry at myself. That can happen when there are certain things you’re used to doing naturally, and you have to try to do them a little bit more or think about them a bit more, which can be frustrating.
“It’s a start. It’s going to take a bit of time to get back to where I was, but as long as I’m committed, that’s all I can ask for, and I can tidy those little pieces up.”
The Leicester cup tie is following by a home game against bottom-placed Watford
“They’re big games,” said Longstaff. “Leicester are a very good side. It’ll be tough, and Watford at the weekend, which will be tough again. I’m looking forward to it though. Everyone’s ready to go.”
Everyone is also that little bit closer after a fractious week on Tyneside."We were already a pretty close group," said Longstaff. "We don’t tend to get too high or too low. It’s a massive three points. It gets our season up and running. It’s something to build on now.”