He says they must be at their best against Sporting, who are second from bottom of the table and seven points adrift of safety.
Karanka told AS: "Sporting beat us here last year and their position in the table is dangerous. They will have to come here to win because a draw is not worth anything, the three points are vital to them.
"We know what is at stake. It is a very important game, we did not win our last game at home and cannot fail again."