The hosts missed a couple of glorious chances in the dying moments but the solitary goal in the first period means Warren Joyce's side will meet Chelsea in the play-off final
The 18-year-old Belgian youth international, who signed for the club in July 2013 from PSV, curled home a fantastic effort from 25-yards to lead United to a 1-0 win at Anfield.
With Brendan Rodgers in attendance, the Reds, who finished the league season in 2nd place, struggled to make home advantage count in the one-off match.
The visitors almost got off to the perfect start as James Wilson saw his effort from the edge of the box rebound back off the post after just four minutes.
Tom Lawrence also provided a threat from deep and a couple of surging runs caused problems in the Liverpool backline.
In a competitive first 45 of few chances it took the individual brilliance of Andreas Pereira a couple of minutes before the interval to put Warren Joyce's side into the lead.
The Brazilian-born attacking midfielder collected the ball on the left flank before cutting inside and bending a powerful shot beyond Ryan Fulton.
Though stunned by the goal, the hosts made a good fist of getting back into the game, gradually growing into the contest as the second half wore on.
And although it took until the 82nd minute for the clearest chance to emerge, Kristoffer Peterson headed over from six yards out following a dangerous set-piece.
The pressure was building and moments later substitute Villyan Bijev was presented with an even better chance. Only the Bulgaria youth international somehow contrived to blaze over from just in front of goal with Ben Amos beaten.
It proved to be the chance Liverpool would regret most as United held on to progress to the play-off final where they will meet Chelsea Under-21s.