Scotland midfielder McTominay nodded home his 10th goal in all competitions this season from Giacomo Raspadori's cross in the 72nd minute to save Napoli's blushes after a drab display in northern Italy.
Belgium striker Lukaku has scored 12 times and set up 10 more in Italy's top flight this season and has been key to Napoli's battle with reigning champions Inter for the league title.
The win moved Napoli three points behind leaders Inter Milan, who beat Udinese 2-1, while third-placed Atalanta lost 1-0 at Fiorentina.
Antonio Conte's side are level on 61 points with reigning champions Inter after a largely lacklustre performance which with better finishing would have still been enough for victory on the Venetian Lagoon.
Antonio Conte's Napoli climbed back to within a point of leaders Inter Milan, who came from two goals down to beat bottom side Monza 3-2 on Saturday.
Reigning champions Inter could cut the gap back to three points when they host Fiorentina on Monday.
Belgium striker Lukaku nodded home his eighth Napoli goal from Andre-Frank Zambo Anguissa's perfect cross in the 78th minute to give Antonio Conte's team a statement win in Bergamo.
Georgia striker Khvicha Kvaratskhelia on Friday signed for Paris Saint-Germain for an undisclosed fee from Italian league leaders Napoli, the French champions announced.
Frank Anguissa and Amir Rrahmani scored Napoli's goals in an eight-minute spell in the first half before Andrea Pinamonti cut the deficit early in the second period.
Giacomo Raspadori smashed in the stoppage time decider to claim a huge win at Juve which pushed Napoli 17 points clear at the top of the table, sweeping aside disappointment at being knocked out of the Champions League by AC Milan midweek.
Leaders Napoli were held to a goalless home draw by relegation-threatened Hellas Verona in Serie A on Saturday as Luciano Spalletti's side finished winless for the third time in their last four games in all competitions.
Ismael Bennacer gave AC Milan a slim advantage in their all-Italian Champions League quarterfinal against Napoli with the only goal in Wednesday's tight 1-0 first-leg win.
Napoli took another step closer to ending their 33-year wait to win the Serie A title after they came away from Lecce with a 2-1 win on Friday thanks to a bizarre own goal from Lecce defender Antonino Gallo.
Juventus have been ordered to close part of their stadium for their next home Serie A match against Napoli by an Italian sports judge after Inter Milan striker Romelu Lukaku was racially abused during Tuesday's Coppa Italia semi-final.
Milan have 51 points, one ahead of fourth-placed Inter Milan, and are four behind second-placed Lazio. Napoli are 20 points clear at the top
The face of a young Diego Armando Maradona gazes down from a mural in Naples' Miracles Square, watching the city gear up for a celebration it hasn't enjoyed since the Argentine soccer star was at his peak more than 30 years ago.
While the fans of Italian football continue to struggle to convince their peers from the English Premier League and LaLiga about the relevance of Serie A in European footballing hierarchy, it is with a renewed optimism that they can now boast about Napoli – the new entertainers of European football.
Serie A leaders Napoli thrashed Torino 4-0 away with a double from Victor Oshimen and goals from Khvicha Kvaratskhelia and Tanguy Ndombele on Sunday to move closer to the title.
Eintracht Frankfurt coach Oliver Glasner said Wednesday that Napoli are on the same level as Bayern Munich after his team were knocked out of the Champions League by Serie A's runaway leaders.
