The Belgium international was on the pitch for two minutes before rounding out Sunday's 4-1 win over Bournemouth and had only been on for 38 seconds when Martin Dubravka spilled Marcus Rashford's free-kick in his direction.
Rashford continued a purple patch in front of goal 10 minutes from time, meaning a fourth win in as many outings at the start of his reign for caretaker manager Ole Gunnar Solskjaer – a feat only previously achieved by club great Matt Busby.
Phil Jones, into the Manchester United defence for the suspended Eric Bailly, made a last-ditch tackle on Salomon Rondon to atone for his own absent-minded play in the 15th minute.
Newcastle winger Christian Atsu spurned a trio of half chances – something that seemed dangerously wasteful when Dubravka had to be quickly off his line to thwart Rashford and Juan Mata.
In between those openings for his team-mates, Anthony Martial sliced onto the roof of the net, which is where Rondon's 34th-minute header also ended up.
Newcastle substitute Jonjo Shelvey struck a wonderful pass to unlock Manchester United with the outside of his right foot in the 60th minute, with Luke Shaw scampering back to deny Ayoze Perez.
But one of the hosts' most valuable performers then let them down and Lukaku was on hand to pounce on Dubravka's error.
Shelvey showcased the ugly side of his game by raking his studs down Paul Pogba's leg – inexplicably escaping any punishment beyond a foul.
Newcastle left themselves exposed for Lukaku and fellow substitute Alexis Sanchez to give Rashford a simple finish and, in stoppage time, Pogba rounded Dubravka on another counter-attack but fired into the side netting.
United close in on top four as Newcastle worries mount
The free-scoring enjoyment of Solskjaer's initial games could not be replicated against a disciplined Newcastle backline but United showed pleasing patience and ruthlessness after half-time. They are three points behind Arsenal in fifth and now just six shy of Chelsea in the final Champions League spot. Maurizio Sarri's side drew 0-0 with third-bottom Southampton, who are within two points of Newcastle in 15th. Crystal Palace and Burnley – either side of Rafael Benitez's team in the standings – were both victorious on Wednesday.
Confidence coursing through Rashford
Pogba was named Manchester United player of the month earlier on Wednesday and the France star's performances have certainly hogged headlines in the wake of his uneasy alliance with Jose Mourinho being put out of its misery. Rashford is perhaps entitled to feel hard done by, having consistently impressed under Solskjaer after being one of the few who did not appear to down tools during Mourinho's final weeks in charge. The free-kick Dubravka decisively spilled was not the first he struck with aplomb and his late goal means the 20-year-old now has five goals in his past seven appearances.
