The young Magpies have found consistency hard to come by this term, having won three and lost three of their seven league fixtures so far.
Peter Beardsley's side suffered a disappointing 4-1 defeat at Wolverhampton Wanderers last time out, Longstaff himself netting their only goal from the penalty spot.
United were 1-0 behind after just 40 seconds at Molineux, their three-game unbeaten sequence brought crashing to an end in the Black Country.
And, with a third of the season already complete, Longstaff feels the time has come for the young Magpies to display their true colours.
"We don't want to be one of those sides who win one and lose one - we don't want to be a hit-and-miss team," he told nufc.co.uk. "We need to find an identity in terms of being a side who are hard to beat.
"Even if we don't win, we need to make sure we don't lose. We have to try and put a little streak together where we go unbeaten for a while, even if it's a case of winning a few and then drawing one."
An ever-present for the Under-23s so far, Longstaff took his tally for the season to four at Molineux, converting from 12 yards after Jamie Holmes was fouled.
The Magpies were already 2-0 down by that point, however, and Longstaff admitted that the damage had been done early on against a Wolves side coached by former United midfielder Scott Sellars.
"The poor start was 100 per cent the reason we lost," he said. "It's always tough coming back, but especially when you're 2-0 down inside the first ten minutes.
"For them to score after 40 seconds wasn't great and it probably set the tone for the game.
"We paid the price for the mistakes that we made by losing the game quite heavily, even though there were times when we played pretty well."
Monday's clash is the first of four in the space of a fortnight for the Under-23s, Fulham currently a point and a place worse off than United.
It represents another long-haul trip for Longstaff and his teammates, but one the young midfielder is relishing.
"I know it sounds like a cliché, but all of the lads love being around each other, so we're looking forward to going there," he said. "We all get along off the pitch, and that can only help us on it.
"Fulham look a pretty good side. We watched a couple of clips of them on Friday. They have a few big lads but they also like to play, like us, and when everyone is trying to play it makes for a better game.
"If we can go there and play to our strengths and do the things we know we can do well, there's no reason why we can't pick up three points."
