Lazio have had a disappointing opening to the domestic season, with one win and three defeats in their first four Serie A games, and are languishing in 15th place.
"I don't know if it’s the best moment to face Atletico, but I know we must earn a good result against the best team in the group on paper," Sarri told reporters on Monday.
"I expect to see the team (Lazio) continue improving, without the flaws that didn't let us get the results we wanted."
By contrast, Atletico had a good start to the domestic campaign, although they did suffer their first LaLiga loss of the season in a 3-0 reverse at Valencia on Saturday, leaving them in seventh spot with seven points from four games.
While Lazio have struggled in Serie A so far this term, the 64-year-old Sarri emphasised the need for a different perspective regarding Europe's showcase competition.
"The Champions League is different. It’s the most important club competition in the world. It must be an honour to play in it and fight for every ball."
Lazio also face Feyenoord and Celtic in Group E.
"We must focus on one game at a time and we need a positive result tomorrow. I partially know Feyenoord but Celtic not so much," Sarri added.
Atletico manager Diego Simeone (53) said Lazio would present a tough challenge at Stadio Olimpico.
"We face tomorrow's match with a lot of enthusiasm and as a big challenge," he said.
"I really like Sarri's style of play; I've always liked it. We've taken many things from his game to improve ourselves."
Simeone, who played for Lazio when they last became Italian champions in 2000, said it will be an emotional return to the Olimpico.
"The extraordinary years of my career come to mind, with a fanbase that loved me from day one," he said. "The affection that exists between us goes beyond an ovation tomorrow on the pitch."
