The assistant coach was in awe of the Frenchman's overall performance and felt his goals were his just deserts, before issuing a rallying cry about los Blancos' title defence
The France international played a key role in the demolition of the Basques, breaking his Liga goalscoring duck for the season with a deflected effort early on, before curling home a second and providing an assist for Mesut Ozil.
"[Benzema] gives us vision, support and movement, and his [second] goal made [his display] perfect. Today he was focused and clinical," he told reporters after the match.
Karanka then reflected on the manner of their victory at the Santiago Bernabeu and hailed his troops for averting the threat of complacency after temporarily letting Athletic back into the match.
"The team maintained their intensity until the end. We could have let Athletic make it 3-2 after they pulled one back, but we talked about it [at half-time].
"In the second half, we were better defensively and then more goals came. This league does not allow setbacks so we must continue on this path.
"[The eight-point deficit] is what it is. We know we cannot fail. We are always playing after [Barcelona] so we must always win."