The Spaniard helped the Blues earn a comfortable 3-0 victory at the Veltins-Arena as they moved to the top of Group E on goal difference.
"I think he's always been a world-class striker," he told Chelsea's official website. "When you are that, eyes are always on you and we saw with his sharpness and his two goals, he has contributed this season.
"We're lucky we have Fernando, Samuel Eto'o and Demba Ba. It's good to have that competition up there but great to see Fernando get his two goals."
Chelsea's Premier League campaign continues on Sunday when they entertain Manchester City at Stamford Bridge.