Luiz received a yellow card for hitting Reeves in the jaw with his shoulder in the final few minutes of the Blues' 4-0 win over the Bees on Sunday.
Brentford manager Uwe Rosler has revealed that Luiz came into the away dressing room after the match and said sorry to a dazed Reeves.
He told The Guardian: "I have not seen the situation clearly enough to make a statement now but the player (Luiz) came into our dressing room and he spoke to Jake, which is a fantastic gesture. He apologised.
"Jake is fine, he wanted to continue but our medical staff said it was better that he didn't.
"Was he concussed? Yes, definitely."
Brentford finished the match with 10 men after Reeves went down hurt as Chelsea progressed in the FA Cup to secure a fifth-round tie against Middlesbrough.