The Spaniards lost 5-0 at the Nou Camp to concede their second loss of the campaign.
Atletico are aware they will have to show a vast improvement if they are to end Rennes' five-game winning home streak in Europe and record their second successive triumph in Europa League Group I.
"We have already put Saturday's defeat behind us," said Lopez, who came off the bench in his team's 2-0 triumph over Celtic in their group opener.
"Barca were the better side and made their most of their chances.
"But we have been working hard to focus on Thursday's game and try to avoid making the same mistakes we did against Barca.
"It's going to be a delicate game at Rennes as we are playing on the road and we know that each rival poses a different challenge.
"Rennes is a physical side and we have to play our game from the first minute to the last and try to bring home three points."
Atletico coach Gregoria Manzano has opted to rest several players with a view on the weekend game against Sevilla, leaving captain Antonio Lopez, Joel, Silvio and Koke back home in Madrid.
Rivals Rennes go into the encounter at the Route de Lorient stadium on the back of Sunday's 1-1 home draw to St Etienne.
The result left Frederic Antonetti's side in fifth place in Ligue 1, two points behind the leaders.
"My team is finding it difficult to play every three days," said Antonetti. "We didn't impose our game against St Etienne and we paid the price.
"Now we must turn the page and focus on another difficult game against Atletico."
Rennes, who lost 2-1 at Udinese in their opening group match, are unbeaten in seven European home games.
The hosts will be without Ghanaian midfielder John Boye, who sustained a muscular injury at the weekend.
"This is a setback for us as John will be sidelined for at least three weeks," Antonetti said.