The Belgian made it 14 goals in 18 Serie A matches with a brace in the opening 45 minutes.
The first was a sublime solo goal in which Lukaku carried the ball from inside his own half all the way to Napoli’s area before firing home.
Then, in the 33rd minute, Napoli goalkeeper Alex Meret couldn’t hold onto Lukaku’s shot and the hosts were suddenly 2-0 down.
The 26-year-old only managed 12 goals in 32 Premier League games for Manchester United last season.
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But he’s back to his best in Serie A and is back among the hottest strikers in Europe.
Lukaku's first goal
Lukaku's second goal
Lukaku was sold by Man United following a disappointing campaign in which he netted just 15 goals in all competitions.
During an interview with Belgian newspaper Het Belang van Limburg in November, the striker admitted that Ole Gunnar Solskjaer wanted him to stay at Old Trafford - but he was determined to move on.
"Solskjaer has my eternal respect. He understood me when I said in March that it was time for me to leave England," Lukaku said, per Sport Witness.
"I have been in the Premier League as an 18-year-old boy, and now I am 26. Solskjaer wanted to keep me, but I was ready for something new."
And Lukaku faced no problems settling at his new club.
"I have had a good relationship with trainer Antonio Conte for three years, and I know exactly what he expects of me," he added.
"The first two weeks at Inter I worked very hard with the group and when my teammates got some rest, I trained individually.
"Thanks to my football intelligence, I integrated easily. We play in a system with two strikers, and that suits me. Anyway, I already knew my team-mates in advance."
The Belgium international is happy and back to his best.
The more he scores for Inter, the more it hurts those at United.News Now - Sport News