The Liga champions' title defence in Spain suffered a further blow on Saturday when they were held to a 2-2 draw away at lowly Levante, Giampaolo Pazzini marking his debut for the hosts with a dramatic late equaliser.
Unlikely to make up a massive 18-point gap on runaway leaders Barcelona in the league and already out of the Copa del Rey after a shock defeat to Leganes, Zinedine Zidane's side will see Europe as their best chance to claim silverware this season to add to August's Supercopa de Espana title.
However, Casemiro acknowledged the mistakes made against Levante have to be eradicated if Madrid hope to progress past PSG and lift the Champions League trophy for a third straight year.
"We made two errors and we let in two goals - that's football," the Brazilian midfielder told beIN SPORTS in the immediate aftermath of the Levante game.
"If you make errors at the back, the opponent has the quality to punish you.
"We started playing well, with control of the game, but then we made those two errors.
"We have to think game by game, but it's clear we can't make those mistakes against PSG."