The Brazilian moved to Stamford Bridge for £26m just over a year ago after four years in Lisbon.
Luiz, 24, said that although he still has friends at the Estadio da Luz he will be ready to fight when the players cross the white lines.
"They'll be my mates right up to the first whistle but, after that, Benfica and Chelsea are going to be adversaries and both are going to fight almost to the death to win," said Luiz.
"Everybody knows I love Benfica and respect Benfica but I won't make it easy for them. I'm not going to leave them any space to win."
Benfica signed Luiz from Brazilian side Vitoria in 2007.