The young Magpies face the Royals at the Madejski Stadium on Friday night (kick-off 7pm BST) after finishing in fourth place of Division Two - the third play-off spot.
United's youngsters have tasted success over Reading in their previous two meetings this term - earning a 2-0 league win at Wycombe Wanderers' Adams Park before a 2-1 triumph in the Premier League Cup last 16 but Longelo believes the squad must take a careful approach ahead of a third clash.
Longelo told nufc.co.uk: "We know it's a very important game for the group because we know if we can beat Reading, we have a huge shot of getting promoted. It's a must win game for us.
"It will be difficult in playing away from home but, with the mentality that we have, we like tough challenges so we'll take it on board and what the manager tells us to face those no matter what they will be.
"It gives us a lot of confidence but, at the same time, it probably gives them confidence to come back and try and beat us too. We can't take the game lightly and need to go there with a strong mentality.
"I think we've had a very good season and it's all down to the group of players that we have. We have a very good team understanding and everybody gets along with each other really well on the pitch as well as off the field. That has helped a lot with the team and is one of the reasons why we have been able to do very well."
After suffering a 5-0 home defeat against their Manchester United counterparts, Newcastle have achieved an impressive 3-0 away victory against Leeds United along with rounding off their league campaign with 1-0 triumph at North East neighbours Middlesbrough.
The victory on Teesside saw United's youngsters amass 40 points - their highest tally since the formation of Premier League 2 and the 19-year-old is hopeful that the young Magpies can reap rewards from an impressive campaign - in which the squad have reached the Premier League Cup final and enjoyed an eye-catching run to the Checkatrade Trophy knockout stages.
Longelo added: "It would mean a lot to everyone if we could get promoted. We have worked hard ever since pre-season as we knew that was the goal. Now, we're very close and everyone would be very happy if we were able to fulfil that dream.
"We're also in the Premier League Cup final and I don't think anyone expected us to go so far in the competition but that shows the team mentality we have. We want to keep winning games whatever comes our way.
"Everyone went home and reviewed the game, thinking to themselves that it wasn't easy. We had to come back against Leeds and show what we were made of. We did that and showed great character to bounce back quickly, winning 3-0 and keeping a clean sheet.
"It's a good advantage. It's always good to keep a clean sheet because it gives us more belief and self-confidence in knowing we can defend as well as attack. That should give us a strong boost ahead of the game in the play-offs."
