The Belgian was on the scoresheet with the third against his former side as United moved level on points and goal difference with rivals Manchester City at the top of the Premier League table.
"We could have scored more I think in the first half. We weren't clinical enough," Lukaku told Sky Sports News afterwards. "The first 25 minutes I think we started really well but then Everton created a lot of problems in midfield.
"The manager gave us a few tips on how to deal with it in the second half. Everton had a lot of control but in the end we made the difference.
"I'm happy with the win and that we stay top of the league. That's the objective for me this season."
Asked if United had been incentivised to score more after City put six past Watford yesterday, Lukaku added: "We don't really look at opponents, we try to focus on ourselves and try to improve game after game. We know that we can play better than we played today."
Next up for United is a home encounter with Burton Albion in the EFL Cup on Wednesday night, ahead of a trip to Southampton in the league on Saturday.