It looked as if the Magpies were heading towards a stalemate draw before Almirón sent the St. James' Park into uproar with a late volley in front of the Gallowgate End - ending a run of 27 Premier League games without a goal.
The three points against the Eagles sends Steve Bruce's side into the top half of the Premier League (ninth) and Longstaff - a second-half substitute - was pleased to pick up maximum points along with keeping a fifth clean sheet of the season.
The midfielder told nufc.co.uk: "Everybody's buzzing in the changing room and everyone is over the moon for Miggy (Almirón) to get off the mark. There's been a lot of people from the outside getting on him but we have always believed in him in the changing room and we know how important he is to us.
"Everyone is delighted with the three points and it does make it better that Miggy has scored the winning goal!
"I was over the moon for him! It was such an massive goal after we were under the cosh a little bit in the second half. When he does that, it made a huge difference for us and we know how important he is for our team and what people on the outside don't see. He deserved his goal after working so hard every single week.
"Palace have got a lot of good forward players and some that are very dangerous who can get at you one versus one which makes them a threat at all times. I have to say the back five and Martin (Dúbravka) were outstanding, particularly Martin.
"He made two big saves to keep us in the game and I think he's probably the top goalkeeper outside of the top six. We're lucky to have him and once again he's proved his worth."
The victory extends United's unbeaten home record to eight matches, with the Magpies last losing on Tyneside against Arsenal on the opening day of the season.
With a trip to Manchester United on Boxing Day followed by United's final clash of 2019 at home to Everton, Longstaff was pleased with the response from last weekend's defeat to Burnley and claim a third wins in the Magpies' last four league matches.
The 22-year-old added: "The dressing room is really close, players and staff. Last weekend was disappointing and we knew we were missing a few lads but they have came back today and made the difference. It's a squad game and whether you're on the bench or starting, you've got to be ready at all times.
"I was disappointed not to start the game today but you've got to go about it the right way. I came on and tried to do my bit for the team and it's a massive three points. It's a sign of a good team when you don't play particularly well.
"There's a lot of confidence in the group. A lot of people wrote us off and we have heard it whether we'll admit it or not. There's still a long way to go but I'd say things are pretty positive at the minute.
"It's good to bounce back after last weekend. I think winning breeds confidence and makes everything a lot better and makes the place a lot happier to be around. We know Man United are on a good run of form and know it will be a really tough game.
"Like I've said, we are a quietly confident bunch, a close knit and we know if we can go there and be hard to beat again, we'll make it tough for them. Hopefully, we can pop up with a goal like Matty (Longstaff) did the last time we played them!"
