one-twos with Larnell Cole and Tunnicliffe dragged wide after being well found by Tyler Blackett.
The second half started with the Reds looking dangerous but Joshua King could only clip against the outside of a post after some poor defending allowed him to chase down a Petrucci pass. Instead, Macheda regained the lead with a diving header from close range after Petrucci nodded Cole's superb cross into the danger zone.
Lingard skied a chance following good work by Tunnicliffe but there were less openings at both ends, even if Maddison did work Johnstone with a powerful shot. Henriquez's introduction added an extra buzz to proceedings and he managed a shot on target after one mazy dribble.
Johnstone made a top-drawer save to prevent Hooper's drive from arrowing into the top corner but United hit back with King wasting a decent opportunity after more good work by Tunnicliffe and Henriquez's header forcing an excellent tip-over by Alnwick.
With time running out, Hooper again brought the best out of Johnstone but there was late joy for Henriquez when he nodded in King's inviting chip following yet more progressive play by Tunnicliffe. Six goals, plenty of entertainment and a satisying third successive win for Warren Joyce's side in the new Under-21 league.
United: Johnstone, Cole (Henriquez 68), Thorpe, Fornasier, Blackett (Veseli 46); Brady, Tunnicliffe, Petrucci, Lingard, King (McGinty 92); Macheda. Subs not used: Gollini, James.