Juergen Klopp's men downed the reigning La Liga champions 2-1 on Wednesday, as Robert Lewandowski and Marcel Schmelzer scored the goals to boost Dortmund's hopes of qualification.

Portuguese international Pepe made a terrible mistake to gift their opponents the opener, with his limp pass allowing Lewandowski a passage to goal.

But Casillas preferred to turn the spotlight on Real's weaker left flank rather than blaming individuals

"Pepe played a superb game and you cannot judge him by that goal. We've been very good in defence and set pieces, but they (took advantage of) our left wing," the goalkeeper told Goal.com.

Casillas acknowledged the fair result, saying that 'the very best of Dortmund has taken to the field' and the 2-1 win was deserved.

Despite the loss, the Real 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," he said.