Mata will line up against his former side Valencia when his new club Chelsea arrive at the Mestalla for their Group E Champions League clash.
Spanish star Mata only left Valencia in the summer to move to Stamford Bridge in a plus £20million deal and Hernandez admits it will be a special moment for his former team-mate to make his return so soon after leaving.
"For Juan it will be special to return to what was his home for many years. But on the pitch there are no friends," he told The Sun
"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."