Mata is set to return to his old stamping ground and Hernandez insists there will be no love lost between the pair tomorrow.
He said: "For Juan it will be special to return to what was his home for many years. But on the pitch there are no friends. We have to fight to the death against Chelsea, beat them and then after the game I will wish him the best.
"I speak to him a lot and I am eager to face him in a competition like the Champions League.
"We know it will be difficult. They are one of the best teams in Europe. But we've shown in the past we can win these games and we will go out to do that."