Rafael Benitez's side were held to a 2-2 draw with Doncaster Rovers last week in a game which they could and should have won, given their personnel.
However, Newcastle bounced back in style by thumping Belgian side Lokeren 4-0, with the "excellent" Ayoze Perez netting twice on Saturday.
The North-East club are odds-on favourites to win the Championship this season and secure an immediate return to the Premier League in the process.
As a result, Newcastle are going to be expected to win the majority of games this season, and the 22-year-old has underlined the importance of staying grounded, suggesting they need to learn from their draw with Rovers last week.
“I was very pleased with the team,” said Lascelles via the Shields Gazette. “I thought the game against Doncaster prepared us for this game. We know we didn’t go there and perform the way we should have.
“Maybe we got a bit complacent, which can’t happen. I think we had the right head on and had the right mentality [against Lokeren].
“We were really sharp, both going forwards and defending. They’re a good team. Ayo was excellent with the goals he scored and we were solid at the back and got the clean sheet.”
Newcastle kick off their Championship campaign with a trip to Fulham on August 5.