The pair arrived at St James’s Park on transfer deadline day.
Almiron joined in a club-record £21million move from Atlanta United, while Barreca – who made his debut in Saturday’s 1-0 defeat to Tottenham Hotspur – was signed on loan from Monaco.
The signing of Almiron could yet persuade manager Rafa Benitez to stay at the club beyond the end of the season.
In the meantime, United’s players are “excited” to see what Almiron and Barreca can add to the team.
“Any new players that come in, you’re excited to see what they’re about, especially the pedigree they’ve both got,” said Longstaff, who made his seventh successive start for the club at Wembley.
“I think everyone’s excited to see what they can do.
“It’s going to be tough, but they’ve got all the lads’ support. I think it gives everyone a boost having new people around.
“As long as we keep working hard, we should be all right.”
Newcastle were beaten by an 83rd-minute strike from Heung-Min Son at Wembley.
United goalkeeper Martin Dubravka let the dipping shot slip under his hands.
The result has left the club 15th in the Premier League table ahead of the February 11 game against Wolverhampton Wanderers at Molineux.
“Everyone’s really disappointed,” said 21-year-old midfielder Longstaff.
“The first half was tough, but we stayed in the game and still created chances.
“I thought we were going to take something from the game. They made a substitution and changed their style a little bit, but I think we coped with it well and one little lapse has cost us.
“We’re disappointed, but there are a lot of positives to build on.
“We played pretty well for a lot of the game and created some good chances. There are positives to take, but everyone’s bitterly disappointed.”
United – who had beaten champions Manchester City at St James’s Park four days earlier – had frustrated Tottenham up to Son’s goal.
The strike took Mauricio Pochettino’s side up to second in the table.
“It (the gameplan) was pretty similar to Man City,” said Longstaff.
“The other team was going to have a lot of the ball. They’ve got a lot of quality. That was the way we set up.
“We did really well up to the goal, and even so sometimes shots from distance go in. We did really well, we kept a good shape and were in the game the whole time.
“At the end we were a little bit of luck away from getting something from the game.”
Newcastle have played five of the top six since Boxing Day, and the club is two points above the relegation zone.
“There were lots of positives to take,” said Longstaff.
“ Obviously, everyone’s bitterly disappointed. It’s been a tough month.
“We’ve played a lot of the top teams, but if someone had said at the start of the month we’d be where we are, we’d have probably taken that.
“With the new signings, the place is a little bit more upbeat. Everyone’s looking up and we’re going into some big, winnable games.
“As long as we listen to the staff and carry out their gameplan, we should be fine.”
Meanwhile, Pochettino, Tottenham’s manager, praised his players after they kept up the pressure on league leaders Liverpool.
“It’s about now praising the squad, the players, to be in the position we are in the Premier League – no-one believed at the beginning of the season that this squad was capable of being there,” said Pochettino.
“Because of our belief, how hard we work, our mentality and everything, we are there. We don’t need to be in some position like today to convince ourselves about our capacity, our potential or possibility to win. We always believe in ourselves.
“We are there because we believe in ourselves. Then it’s if we are capable to fight until the end, to be close and fight until the end to go close to our opponent to be champions.
“We’re there because we deserve to be there. We will see what happens in the future. It’s easy to praise the mentality of the team. In some parts of course we are showing great mentality. But in the end, to win the games, sometimes you need some luck.”