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.
The 24-year-old is now the highest scoring African player in the history of Serie A after overtaking George Weah and lapped up the applause of Napoli fans as the team did their post-match lap of honour
Napoli's champions returned home Friday after winning their first Serie A title in 33 years, greeted at their training camp by hundreds of jubilant fans following a raucous night of partying in Naples.
More than 200 people ended up in hospital overnight for injuries sustained during the celebrations, La Stampa said, from knife wounds to burns from firecrackers and asthma attacks caused by inhaling smoke from flares.
Retired midfielder Alemao, who won Serie A with Napoli in 1990 alongside late legend Diego Maradona, says he regrets his friend is not alive to celebrate the team's return to glory 33 years later.
Victor Osimhen said Thursday that he will never forget winning Napoli's first Serie A title in over three decades after his goal finally pushed southern Italy's biggest club over the line.
Napoli will have to wait at least one more day to be crowned Italian champions after Lazio beat Sassuolo 2-0 on Wednesday to reclaim second place in Serie A.
Leaders Napoli were held to a 1-1 home draw by Salernitana in Serie A on Sunday, extending the wait of Luciano Spalletti's side to seal their first title in 33 years.
Inter Milan came from a goal down to win 3-1 at home against Lazio in Serie A on Sunday, giving Napoli the opportunity to seal their first title since the Diego Maradona era this weekend.
In order to win the league a record-breaking six matches before the end of the season, Napoli have to hope closest challengers Lazio do not to win at Inter Milan in Sunday's early kick-off
Thousands of joyous Napoli supporters greeted the team early Monday as they returned from their victory over Juventus, which brought the prospect of a historic Serie A title within touching distance.
