Last season's finalists are heavy favourites to overcome the Portuguese giants.
"We do believe we can go through to the next stage," Casillas said.
"That is our mentality. There is no other possible thought.
"It is not a tie between a big and a small side. And we should not look back to what happened last season when they knocked us out of the competition.
"We have a huge respect for Liverpool and every other club involved in the Champions League. But I believe the other teams also respect Porto. They should know we have top players capable of facing any other side."
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