The Spanish international insists his teammate cannot be held responsible for Los Merengues' bleak performance at the Westfalenstadion, insisting Dortmund was simply superior.
Jurgen Klopp's men downed the reigning La Liga champions 2-1 on Wednesday, as Robert Lewandowski and Marcel Schmelzer scored the goals to boost Die Schwarzgelben's hopes of qualification.
Portguese international Pepe made a horrible mistake to gift their opponents the opening goal, as his limp pass allowed Lewandowski through on goal to finish. But Casillas preferred to turn the spotlight on Los Blancos' weaker left flank rather than apportion individual blame.
"Pepe played a superb game, and you cannot judge him by that goal. We've been very good in defense and set pieces, but they [took advantage of] our left wing," the goalkeeper told Goal.com.
Casillas acknowledged the fair result, stating that "the very best of Dortmund has taken to the field" and the 2-1 win was deserved.
Despite the loss, the Madrid stalwart remains optimistic of progression to the last 16 of Europe's elite club competition.
"Ajax have beaten Manchester City, so the second leg match [against Dortmund] will be very important. [It's] vital to get the victory to take the three points," Casillas said.